No. Stop it. Just multiply them together. This is useless.
this post was submitted on 20 Oct 2025
775 points (98.1% liked)
Science Memes
17142 readers
2847 users here now
Welcome to c/science_memes @ Mander.xyz!
A place for majestic STEMLORD peacocking, as well as memes about the realities of working in a lab.
Rules
- Don't throw mud. Behave like an intellectual and remember the human.
- Keep it rooted (on topic).
- No spam.
- Infographics welcome, get schooled.
This is a science community. We use the Dawkins definition of meme.
Research Committee
Other Mander Communities
Science and Research
Biology and Life Sciences
- !abiogenesis@mander.xyz
- !animal-behavior@mander.xyz
- !anthropology@mander.xyz
- !arachnology@mander.xyz
- !balconygardening@slrpnk.net
- !biodiversity@mander.xyz
- !biology@mander.xyz
- !biophysics@mander.xyz
- !botany@mander.xyz
- !ecology@mander.xyz
- !entomology@mander.xyz
- !fermentation@mander.xyz
- !herpetology@mander.xyz
- !houseplants@mander.xyz
- !medicine@mander.xyz
- !microscopy@mander.xyz
- !mycology@mander.xyz
- !nudibranchs@mander.xyz
- !nutrition@mander.xyz
- !palaeoecology@mander.xyz
- !palaeontology@mander.xyz
- !photosynthesis@mander.xyz
- !plantid@mander.xyz
- !plants@mander.xyz
- !reptiles and amphibians@mander.xyz
Physical Sciences
- !astronomy@mander.xyz
- !chemistry@mander.xyz
- !earthscience@mander.xyz
- !geography@mander.xyz
- !geospatial@mander.xyz
- !nuclear@mander.xyz
- !physics@mander.xyz
- !quantum-computing@mander.xyz
- !spectroscopy@mander.xyz
Humanities and Social Sciences
Practical and Applied Sciences
- !exercise-and sports-science@mander.xyz
- !gardening@mander.xyz
- !self sufficiency@mander.xyz
- !soilscience@slrpnk.net
- !terrariums@mander.xyz
- !timelapse@mander.xyz
Memes
Miscellaneous
founded 2 years ago
MODERATORS
This only works in small cases.
I also remember a somewhat sinister question: Using a knife, divide 3 apples among 4 people with the less possible cuts. 💀
4% is 1/25. 75 is 3*25. so the answer is 3!
3! = 3*2*1 = 6
That's the same order of magnitude, so still correct!
Ah, a man of science!
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*75/100
Formulated like this it's really obvious why the method up there works too
load more comments
(1 replies)
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.
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.
Speed limits are trickier than structural safety margins because of several factors:
- In some areas, particularly remote areas, the process isn't very well defined. Sometimes the speed limit will be set by one guy who just felt like that was fine. Doesn't even have to be an engineer really.
- Standards evolve over time (trending towards lower speed limits) but speed limits only change when a tragedy or major road renovation happens. Where I live there's sometimes a 40 km/h spread on posted speed limits for similar roads depending on whether they were rebuilt last year or 50 years ago.
- Car culture means drivers hold a ton of political power. There are a myriad of traffic devices that cannot be built not because of practical or financial constraints, only because they would "inconvenience drivers". Lower speed limits are often one of those. People complain so the government backs down despite engineering recommendations.
- A driver is always liable if they drive too fast for the conditions, not the traffic engineer. That goes to the previous point, with zero penalty for not sticking to the sensible engineering choice, political pressure easily wins out. Hard to argue against a work order when the person signing off on it cannot be sued for negligence.
The upshot is speed limits in my local experience have a lot more to do with the municipality/region's political climate than engineering standards and safety factors. Sometimes I feel like I could safely go 2x, sometimes the limit is 90 km/h on a two-way one lane road with 30 m of visibility where 30 km/h feels like I'm pushing it.
That’s because elevators use counter weights usually equal to the weight of the car and half the occupancy load so that it takes less energy to lift it and if it falls for any reason it won’t hit the bottom as long as the counter weights are still attached. The occupancy load is determined by the counter weighting system not the cable load capacity.
load more comments
(1 replies)
load more comments
(1 replies)
Can I also smoke two joints before I smoke two joints, and then smoke two more?
load more comments
(5 replies)
load more comments
(1 replies)
Mine brain just does 0.75 × 4.
Thought process was...
- Get 1% = 0.75
- Double it = 1.5
- Double it = 3
load more comments
(8 replies)
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
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.
Answer
One…
A-two
A-three. Three licks to the Tootsie Roll center of a Tootsie Pop.
load more comments
(1 replies)
load more comments
(1 replies)
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
load more comments
(1 replies)
I'd do 4%=1/25, 75/25=3.
That's a neat trick but also 4% of 75 = (1% of 75) * 4 = 0.75 * 4 = 1.5 * 2 = 3
639% of 1282.5
Is it weird that I just went
start: 75
to actual decimal: .75
*2*2: 1.5 -> 3
load more comments
(1 replies)
view more: next ›