Skip Navigation

Science Memes @mander.xyz

Stamets @lemmy.dbzer0.com

1w ago

Fucking math...

69 comments

4% is 4 in a hundred
75 is 3/4 of a hundred
4% of 75 is 3
I also remember a somewhat sinister question: Using a knife, divide 3 apples among 4 people with the less possible cuts. 💀
- Is that the one that goes: cut one person, give the three survivors an apple each?
I only learned this a few months ago here on Lemmy and still don't believe it. It's magic. You think you're going to "get it" (trick it) this time, but nope! Still works somehow!
- What’s funny is that if someone said (a * b) * c is the same as (c * b) * a, you’d probably say “of course it is”.
  Same trick here if you show it as (4* 1/100) * 75 rewritten as (75 * 1/100) * 4.
75% = 3/4
(3/4) * 4 = 3
Yeah, this will be so helpful when I'm trying to figure out 20% of 36.23!
Look, move the decimal one to the left, you've now got 10%. Double that and you've got 20%. How often are y'all trying to find out percentages of nice even numbers like that?
- See I love quick and dirty rules to get close enough through estimation for whatever I'm mental mathing, because if I need exact numbers I'm turning to a computation device
  20% of 36.23 I'd be going "okay 20% of 10 is 2, 3 10s in 36 so 3x2=6, and 6.23 is pretty close to half 10 and half 2 (from my previous 20% of 10 calculation) is 1 so 20% of 36.23 is slightly more than 7"
  36.23% of 20 I'd be going "30% of 10 is 3, 2 10s in 20 so 2x3=6, 6.23% is close to 5 so half of 3 is 1.5, 6+1.5=7.5 so 36.23% of 20 is a bit more than 7.5"
  Now which is closer to correct? Ehh I'm not sure I haven't used a calculator yet, but I'm mental mathing so chances are my estimation got me close enough that I can just round to whichever direction is safer for errors and call it good. Usually I'm mental mathing to figure out splitting a bill, a tip or to double check some machine computed math that looks wrong, and none of those call for perfect precision, just getting close enough that it doesn't matter
  
  36.23 × 20% = 36.23 × 2 ÷ 10 = 7.246
4% is 1/25. 75 is 3*25. so the answer is 3!
- 3! = 321 = 6
  
  That's the same order of magnitude, so still correct!
No. Stop it. Just multiply them together. This is useless.
This only works in small cases.
My engineering brain says it's 3.25.
4% is ~ 5%. 10% of 75 is 7.5. To get the 5% I have to divide it by 2, so 4% of 75 is close to 3.25. I will have to multiply it with some safety coefficient at the end, so the exact value doesn't matter.
- Yuck. 75 is such a clean number to work with in percentages, it's 3/4 of a hundred
  3/4 of 4 is then easy
- That’s why you can always double the maximum limits engineers give.
  60 mph roadway?
  I can do 120 on it no problem.
  Eight person elevator? Sixteen.
  0.08 BAC? 0.16 easy peasy.
  
  Yes, in elevators usually one cable could hold far more than the full weight, then they add 5 more for the safety.
  For rail speed limits this is the exact way they calculate it. For road speed limits they consider braking distance, which grows by the square of your speed, so if you go 120 on 60 road, you will need 4 times the distance to stop. I wrote 1.5 as a safety factor, not 4, With a 1.5 safety factor you can go by 75 though, but I would use a 1.1 safety there, as in my country the speed cameras are set up that way, you can go +10% of the official speed limit, they only send a cheque if you went even quicker than that.
  
  Can I also smoke two joints before I smoke two joints, and then smoke two more?
  
  Did engineers come up with that last one though?
- I'm confused by this statement, the answer is 3. Why do all these extra steps for a wrong answer?
  
  It's not wrong, it's close enough. And the point it works with more numbers and more type of calculation. Let's calculate 4% of 1243. That's the same as 1243% of 4, right, much easier to calculate by simply changing the 2 numbers... While my method is the same, by simply rounding everything.
  And in engineering you always multiply/divide your results by a 1.5 or 1.25 safety factor, depending on situation. So you don't have to calculate exact results, just close enough. E.g. G is always 10m/s2. π is only 3.14, the other digits doesn't matter.
- That's exactly why we have safety coefficients.
  
  Safety coefficients are for nerds
- One…
  A-two
  A-three. Three licks to the Tootsie Roll center of a Tootsie Pop.
0.04 x 75 == 0.75 x 4 == 75 x 4 x 0.01
- switch the order of the last two terms (the second equality), put the 0.01 in the middle, and it makes a bit more sense when read as calculation steps.
  
  0.04 * 75 = 4 * 0.01 * 75 = 4 * 0.75
  
  That's fair. I intended them to just be a list of equalities
4*75/100
- Formulated like this it's really obvious why the method up there works too
- This is the way
That's a neat trick but also 4% of 75 = (1% of 75) * 4 = 0.75 * 4 = 1.5 * 2 = 3
Ah, joy of commutative algebra.
Wait until you get to noncommutative algebra.. shudders. No one who mastered that monster of a subject is sane in any measure.
- Yay for quaternions and beyond
  
  I encountered those in game dev a while ago. Honestly, fuck them.
I'd do 4%=1/25, 75/25=3.
Is it weird that I just went
start: 75
to actual decimal: .75
2*2: 1.5 -> 3
- Nah we all do weird shit. I did 4x75, then moved from 300. to 3
  
  Meanwhile im the idiot thats like "uh 10% of 75 is 7.5, half it for 5% of 75 is 3.75, 1% of 75 is .75, so its probably 3?"
  Lets pray thats one of the options on the multiple choice. Oh the professor wants me to show my math? Well lets hope he's open to me being an abstract dumbass that is capable of getting the right answer.
  
  I just figured 4% is 1/25 and divided 75/25
- That's how my brain works as well.
Even the first one, 0.75 doubled and then doubled again is hard?
- at this point in my life, as somebody with multiple math degrees, if i ever come across a situation where i need to multiply or divide numbers and one of them is bigger than 12, im going to use a calculator.
  
  Look at Mr big brain over here who learned his times tables. Sure hope nobody takes his lunch money.
  
  Shitty Math Pro Tip: If you ever have to work with numbers larger than 10, convert it to scientific notation and then round to a single significant figure.
  
  ? = 0.04 x 75 ? = (4 x 10^-2) x (7.5 x 10^1) ? ~= (4 x 10^-2) x (8 x 10^1) ? = 4 x 8 x 10^-1 ? = 32 x 10^-1 ? = 3.2 ? ~= 3
  See how easy that is? Here's another one:
  
  ? = 12 x 12 ? = (1.2 x 10^1) x (1.2 x 10^1) ? ~= (1 x 10^1) x (1 x 10^1) ? = 1 x 10^2 ? = 100
  Bam, lock it in.
Why did no one ever teach me this?? Did I miss this day in class? I feel so silly. This is really useful.
- Most teachers will write it off as obvious. Taking a percentage of something is just multiplication and if you actually write it down with multiplication, it is, indeed, obvious:
  
  4*75/100=75*4/100
  And yes, it means you can just multiply 75 by 4 first and then divide by 100.
- Yes and no, other day I was trying to figure out 17% of a number like 65, and I'm like "Oh it's just 65% of 17!" Which really wasn't helpful.
  It works with small numbers on one side tho.
0.75*4
4 over 100
is
X over 75
And there's your butterfly.
Just do (7-5)+(5-4)=3
- i prefer the easier to remember (7 * 4) - (5^(√4))
Asian Superman knows this

69 comments