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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.
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
75% = 3/4
(3/4) * 4 = 3
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639% of 1282.5
0.75*4
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*4/100And 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.
4*75/100Formulated like this it’s really obvious why the method up there works too
This is the way
Asian Superman knows this
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.
Can I also smoke two joints before I smoke two joints, and then smoke two more?
At that point you will have smoked 4% of 100 joints. Hey, math is easy.
How do Europeans even measure weed lol
In grams bro.
That’s trippy man, so do we.
The real question is do they start using ounces for amounts equal to or over 28 grams?
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.
Yeah, as I understand it, the elevator will refuse to move instead of collapse, and hopefully you’re not between floors when it happens because it was close and someone shifted their weight or bounced slightly or they might write a sitcom episode about what happens next (and the reality will be far more boring).
Mine brain just does 0.75 × 4.
Thought process was…
- Get 1% = 0.75
- Double it = 1.5
- Double it = 3
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.
Huh? It’s not “close enough”, it’s exactly accurate. 4% of 75 is 3 exactly. I don’t know where the rest of what you wrote comes from. This post is about pure arithmetic
That’s the stupidest shit I have heard today. You should feel ashamed if you really are an engineer
That’s how engineering is. In civil you can round π=5 for a lot of calculations. In astrophysics I’ve seen e=π=10
Rounding once may be okay but rounding multiple times and that errors add up. Astrophysics?! If im working with wood, i don’t care measuring to 0.1 mm and it might be okay in astrophysics to use 10 for pi, but that doesn’t make guessing your math correct in general.
Maybe we are doing things differently here in germany.
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.
You bit that, you cheated! I saw you.
That’s a neat trick but also 4% of 75 = (1% of 75) * 4 = 0.75 * 4 = 1.5 * 2 = 3
I’d do 4%=1/25, 75/25=3.
