Spyke
lemmy.world

Yeah, I personally find it a lot easier to compute 13% of 17, rather than 17% of 13.

117
lemmy.world

The few times I've seen you on Lemmy, it takes me a few seconds to process your username. Today, I had my favorite misreading yet: pronounced Ivana footerall

54

No I think they specialise in medium or large format, with various filters… doing their own development with funky chemistry.

I don’t see this person as a black and white character. Very creative. Specialising in avant-garde still life maybe.

2

Actually I disagree.

20% is 1/5

100/5=20

10/5=2

5/5=1

3*20=60

7*2=14

1*1=1

60+14+1=75

1

.2*13=2.6

.03*13=0.39

2.6-0.39=2.21

My brain is maybe a weird place 😂

2
pyrereply
lemmy.world

I guess it's easy to miss that m% just means ×m÷100

6% of 50 means 50 × 6 ÷ 100, and because multiplication and division have the same order of operations you can switch them around. so it is also equal to 6 × 50 ÷ 100, in which you can simplify 50 ÷ 100 into 1 ÷ 2, but you could always do that: 50 × 6 ÷ 100 = 1 × 6 ÷ 2.

this "shortcut" however is of extremely limited use. basically for taking a percentage of anything other than things that can very easily divide or multiply 100 like 50 in this example or maybe 10 or 500 it's nearly useless.

16
maccentricreply
sh.itjust.works

Not really tho. If you know 9x6=54 then it’s pretty easy to get to 5.4 from there

7
lemmy.ca

x/100 * y = x * 1/100 * y = x * y/100

I never would have thought of it myself and still feel like I should have thought of it myself.

104
chocratesreply
piefed.world

So 37 % of 38 is equivalent to (37 * 38 ) / 100 ? Which I can almost do in my head.

37 * 4 is 148 I think, so 37 * 40 is 1480. Subtract 74 we have 1406. So we have 14.06?

12
wabassoreply
lemmy.ca

For a while I’ve been meaning to memorize all the two digit squares but of course I then fail to practice them, so I’m not very far along. But that would help you with this too, not to mention you can start doing some square roots too.

For quick and dirty I’d probably just go with 37*38 = 40*40 -> 16% which is kinda close to 14% eh?

4

Oh absolutely, depends on what you are measuring if that isn't close enough

2
db2reply
lemmy.world

You clearly don't have gigs of memes.

11
jbrainsreply
sh.itjust.works

Pretty close to 1/4 of 52, so like 13-ish, but maybe closer to 12.

(13 + 0.25 - 0.53 - 0.53, really. If I had to, I might be able to keep that in my head.)

3
lemmy.world

May as well just get out the calculator app to make sure. My point with those numbers is to just use a calculator. Why 2nd guess anything?

1
jbrainsreply
sh.itjust.works

If "close enough" works, then it's nice to have the skill. Having the skill requires occasionally using it.

Where accuracy is important, since we almost always have a calculator with us now, that's a no-brainer.

Maybe more to the point, though, understanding how percentages work is wise. It's one of the few arithmetic topics that we encounter regularly in life.

In this case, 23% of 53 and 53% of 23 each have their own little trick, depending whether you'd rather overestimate a little with 1/4 of 52 or underestimate a little with half of 24. I find it handy to be able to think that way, especially for example when trying to get out of a taxi and paying cash.

1

Where accuracy is important, since we almost always have a calculator with us now, that’s a no-brainer.

Unfortunately most calculator apps were written by no-brainers who didn't check they had their Maths correct. If you need accuracy then get a name brand like Sharp or Casio.

1

May as well just get out the calculator app to make sure

Calculator apps get order of operations questions wrong because of programmers who were too lazy to check they had their Maths correct

My point with those numbers is to just use a calculator.

My point is avoid calculator apps like the plague. Use name brands like Sharp or Casio.

Why 2nd guess anything?

Guess what the programmer of your calculator app did 😂

1
lemmy.world

My dyscalculic ass trying to remember what 50% of 6 is

46
lemmy.world

I'm gonna track down all my HS math teachers and punch each and every one of them in the mouth.

41
slrpnk.net

Legitimately new knowledge for me that will literally make my work easier. Wish I could give you more than 100% of 1 upvote, OP!

40
startrek.website

That's because multiplication is commutative

taking a percentage of something essentially means multiplying it with a hundreath of the percentage
6% of 50 essentially means 50 * 0.06
or 50 * 6 * 0.01

and since
50 * 6 * 0.01 = 6 * 50 * 0.01
then of course
50 * 0.06 = 6 * 0.5

And we have the above

35

Yeah my brain just sees 5*6, and then I move the decimal. I never understood how people couldn't figure out tips when they wanted 20%.

If you live in the U.S. you do them quite often, multiply by 2. Want 10 multiply by .1... Half that and add it if you want 15. Whatever is easiest at that moment

7
ricecakereply
sh.itjust.works

I prefer to keep it technically correct yet evil and confusing. 6% being a fancy way to write 0.06 or 6 * 1/100 means we can take 6 * 50 * 1/100 and simplify to 300 * 1/100 and then represent that as 300%.

6
bentreply
feddit.dk

Finally a reasonable way to do percentages! Thank you, I'm stealing this

1
Johandeareply
feddit.nu

... in 1 or 2 dimensional number systems, also known as the real (1-dimension) and the complex (2-dimensions) numbers. With quaternions and higher dimensional systems multiplication is not communicative. In fact, the more dimensions you add, another mathematical property is lost.

0
lemmy.world

... okay? Yes? Nobody thought otherwise? Do we now have to clarify every statement about algebra by specifying that we're talking about an algebra over the reals or the complex numbers? Or the polynomials or the p-adic integers, whose multiplications are also commutative?

No one would call these "n-dimensional" number systems either. The algebra for each of these operates in R^1^ and R^2^, respectively, but, like, you would describe their algebras as being over an n-dimensional vector space. It's not wrong, but I don't think "two-dimensional number system" is something you'd hear mathematicians say.

This pedantic aside feels so "I just watched a 3blue1brown video and feel verysmart(TM)" that I don't know what to do with it. It's good to be interested in math, but this ain't it. Everyone knew what they meant.

6

Or, more generally:

Taking X percent of something is the same as multiplying by X/100.

X percent of Y = Y(X/100)=(YX)/100

Y percent of X = X(Y/100)=(XY)/100

(YX)/100=(XY)/100

Percentages are indeed reversible.

6
0x0
lemmy.zip

6/100 * 50 = 50/100 * 6
6 * 1/100 * 50 = 50 * 1/100 * 6

26
lemmy.world

Yes, 47% of 47 is certainly much easier to determine when you look at it as 47% of 47! Genius!

25
rumbareply
lemmy.zip

You can also

Multiply one side, divide the other to get the base number to get the base number to 100.

6% of 50 == 3% of 100 == 3

5% of 20 == 1% of 100 == 1

7% of 50 == 3.5℅ of 100 == 3.5

14% of 200 == 7% of 100 == 7

Edit* as pointed out below, this one was incorrect.

14% of 200 == 28% of 100 == 28

You don't have to stop at percentage either.

25 * 16 = 100 * 4

If you're doing addition, you can add and subtract.

27 + 13 = 20 + 20

Most of the difficulty is in realizing that one number or the other can be brought up to a nice round number, making the equation simple to do in your head. And obviously, just getting to around number on one side doesn't always make it easy on the other side.

I always loved screwing with math problems to make them easier, which is weird, because overall, I don't really care for math.

I also do shit like borrowing a couple of numbers to make the equation easy and then pull them back out.

392 / 4 == (400 - 8) / 4 == (100-2) = 98

376 / 4 == (400-24)/4 =100-6=94

Of course it goes up a level when the remainder isn't evenly divisible. But I still find it's something I can handle in my head.

371 / 4 == (400-29)/4= 100-7¼ =100-7.25=92.75

24
Lemminaryreply
lemmy.world

14% of 200 == 28% of 100 == 28

You can also do 200% of 14, which I find more straightforward.

4

I would argue you're still doing the other side just subliminally. But it is a fair point.

1
lesnout27reply
feddit.org

Why do you have to divide on both sides in your example with 14% of 200?

2

Oh shit, there I go getting along winded and get lost, yep, that should have been multiplied on the other side.

2
lemmy.world

Are you just out here doing common core math on here like it's just some smart trick to make math easier?! You dirty bastard, some conservative is going to see this and think it's a good idea, then they'll just innocently do it in front of their conservative friends and all hell will break loose. Homosexual fornication will suddenly run rampant through the group and those few that can resist the call of the devils numbers will have to cull the rest. You know... Actually... I am not completely against that now that I think about it...

1
rumbareply
lemmy.zip

Rofl, I pre-date common core. I'm not exactly sure to what extent they do all that stuff. I do know that they've got a fuck of a lot of numberlines, like so many number lines. Like they're being paid by the number line.

Back in the 80s and 90s, the teachers would do regular scrolling down arithmetic, and they would mention borrowing numbers and shortcuts. And I took that shit seriously because I did not like sitting down and adding/multiplying numbers. I'd probably spend an extra 30 seconds of problem if I thought I could not have to sit down and write equations.

2
lemmy.world

That's really cool. The borrowing thing is the part of common core that they teach in elementary, and it is the part that enrages conservatives. The point of the homework and practice questions is to repeatedly do the borrowing thing until it becomes automatic and something that they do in their heads all of the time. So, it absolutely enrages the right when kids get the correct answer, but lose points for doing the problem the wrong way.
I was taught to do it the hard way and had to figure out the other tricks for myself much later in life, so now, even after decades of doing it your way, it's more of a struggle than it should be and I just reach for my calculator instead. So I envy you quite a bit.

Keep up the good work with your subversive teaching of common core principles as tips, tricks, cheats and shortcuts. That's actually how they should have branded it, instead of common core, it should have been cheating core, then they would have been all for it.

1
rumbareply
lemmy.zip

There's nothing wrong with calculators.

I wonder if the next generation is going to be reaching for AI for all their English needs.

1
lemmy.world

I haven't sent anything to anyone that wasn't spell checked in more than a decade. So, ya it's going to be a weird future where you just choose the comment you want to respond to and AI fills it out for you.

1

Ohh man, think of training something on your own corpus of writing over the years.

god we could probably more or less do that now...

2

There’s nothing wrong with calculators

There is if you're talking about calculators on phones, etc. Almost all of them give wrong answers to order of operations questions because the programmer didn't bother checking their Maths first. It's so bad that the Windows calculator in Standard mode says 2+3x4=20. Stick to name brands like Sharp and Casio. They have money invested in the success of their products, so they take more care to make sure it's correct!

1

Teaching stats and using percentages now and again, it's not really that useful of a tool most of the time since you aren't usually working with an easy number like 50 (Or 10, 20, 25, etc.).

Usually what I'm doing is breaking it down into manageable parts (factors), so 6% is 5% + 1% which I can usually do in my head regardless of the number (or if 5% hard, work in increments of 1%, which I can multiply by 6).

It's a pretty common strategy to with with factors. Just surprised how many of my students don't know it!

22

Huh. Learning a fun and cool math trick was not on the list of expected outcomes for today, but I'll take it!

20
leminal.space

Most of the people explaining this still make it more complicated than it needs to be, imo.

I just think the symbol "%" algebraically means 0.01

16
kbin.earth

Surprised how many people don’t know this. I suspect it is because people forget that percent literally means per 100 or /100 and the of is standing in for multiplication. So 6% of 50 is 6/100 * 50 or 6 * 50 * 1/100

14

There is no 'M' or 'O' in the statement "six percent of fifty" There also is no exponentiation so I have no idea what you are on about

1
lemmy.today

Yeah, now calculate 6% of 36

Edit: Replies itt make me believe that everyone on Lemmy is a chatbot

10

1% of 36 is .36, and .36 * 6 = 2.16. I'm a fan of this method most of the time, though the one in the OP is good for some math problems.

5
cub Guccireply
lemmy.today

6x3x10 + 6x6, move decimal point 2 places to the left

Wow, that's nice! I like it.

I'm a Maths teacher :-)

I just hope that you are not starting grading your students with the phrase "You're absolutely right!"

2

Wow, that’s nice! I like it

It's called grouping. It's part of the curriculum here. Break the arithmetic into easily calculated groups.

I just hope that you are not starting grading your students with the phrase “You’re absolutely right!”

I'm not in the U.S. no. We still require teachers to be qualified here (Masters)! 😂

1

3.6/2+0.36 = 2.16

(10% of 36 halved, so 5%, plus 1% of 36)

1

I usualy get the value of 1% and then multiple that back to original, so:

1% of 36 is 0.36 (just move 2 spaces) and thats times 6 which is 2.16

1
lemmy.world

This is the coolest trick since I learned about the "quick 20% tip on your tab" method. Take the total bill, move the decimal point one place over to the left, then multiply it by 2 to get 20% gratuity. I will sometimes round down on the change to make it closer to 18%. Also I only really need to do this when I'm drunk and can't maths.

10
lemmy.world

Why would you multiply by 2? 10% is a good tip.

As long as wages are supplemented by customers, the restaurant industry will never pay their employees a fair wage.

-2

10% is in fact, not a good tip, despite your misplaced morals. You think by stiffing servers that restaurants will care? Do you honestly think that will make a difference, or are you just cheap? Build the cost of a tip into what you expect to pay for a meal out or don't eat out. Tip culture sucks but it's the standard and you don't have to like the standard but fix it at the ballot box, not at the dinner table.

6
untorquerreply
lemmy.world

As a former service worker,

  1. fuck you
  2. this is NOT solidarity
  3. bosses won't pay more because you pay less
  4. pay tips in cash. PAY TIPS IN CASH IF POSSIBLE
5
lemmy.world

Why cash? So you can split my $5 with the other 20 staff members? Or so you can hide it from your team? If your boss is taking money from the tip pool paid through card transactions that’s illegal. You are forcing industry problems onto the customer and frankly it’s not my problem. I feel bad for you but all I have are likes and prayers. I’m not gonna let employees who are scared to hold their employers accountable to guilt trip me into giving them my money. 10% is extremely reasonable considering the standard for service in restaurants right now. I can’t remember the last time I didn’t have to go find a staff member to get a refill.

1
untorquerreply
lemmy.world

In restaurant cash isn't always as important. However the boss/power dynamic and employment laws can mean service workers are often left with little to no bargaining power.

Cash is more important for food delivery services so the company doesn't take a majority cut. Though, judging by your statements, I'm guessing you feel insulted by the very idea of tipping for delivery.

1
lemmy.world

“Little to no bargaining power” doesn’t mean anything. You hold your boss accountable to treating you fairly. I met a kid that last year worked in a cash only Vietnamese sandwich shop. Only white kid there. He essentially unionized the place and stopped the owner from stealing their tips.

If a 20 year old can stand up to person like that when he can barely speak the language, I don’t want to hear about how a group of Americans who don’t have to fear of getting deported or send any money back home to feed their family struggle in the work environment.

It really sounds like a crazy story but he did it and probably changed some of those workers lives. And don’t even get me started on delivery fees that for whatever reason the workforce agreed shouldn’t be paid to them.

1
Furbagreply
lemmy.world

I don't like tip culture either, but I'm not going to stiff the guy making federal minimum wage.

If I'm drunk and having a good time, 20% seems fair to me. You can pay whatever you think is fair, I'm not passing judgement on that account.

5

Wise panzer of the lake. There are other wise panzers, but they are malevolent, untrustworthy.

1
lemmy.world

Yeah stuff like that really ain't it. It works in a few use cases, but is objectively wrong and detracts from understanding the topic properly. That's why I teach percentages as the fractions they are. By the time you learn percentages, you already know multiplying fractions is commutative, so the trick still works, and you also understand why.

7

That's pi/100 still. Didn't say it was a rational number, just that it was a fraction. Though I don't see a context where it'd make sense.

1

Yeah stuff like that really ain’t it

Yes it is

It works in a few use cases,

It works in every case where you have multiplying and dividing, including fractions and percentages

but is objectively wrong

No it isn't

detracts from understanding the topic properly

Enhances it actually

That’s why I teach percentages as the fractions they are.

Sounds like you're only teaching as much as you understand. Try understanding more. Students love the tricks that make Maths easier, including this one.

1
lemmy.world

% really just means *1/100 - it doesn't matter where you apply that factor: 6% of 50 = 6% * 50 = 6/100 * 50 = 6*50/100 = 6 * 50/100

4

I was probably on a different continent ^^ Also, the biggest problem explaining math is not making it understandable, but having a motivated learner. Most students that have a bad teacher in school, or are somehow not motivated in school to begin with, will also not be motivated to focus on easier explanations. Technically a basic interest in math should be fostered / encouraged in kids from an early age, and at the latest in elementary school. If that does not work out, many students will have a hard time with it.

You are welcome.

3

50*.06 = 50*6*1/100 = .5*6

But you're still just multiplying 5*6 and then moving the decimal point around. So don't sweat it.

2

maths is the same in both case but I get it, for small numbers this is useful

2