Why YSK: It helps when calculating percentages. If you want to find out what 2% of 20 is, do 20% of 2 instead and you’ll get 0.4. So, 2% of 20 also equals 0.4.
Or, 12% of 25 can be alternatively written out as 25% of 12 instead, which would be 3.
Explanation: Commutative Property. So, 0.07 times 50 will always equal 50 times 0.07
Percentage is just a fancy form of decimal point. I just multiply them together then move decimal digit to the left twice.
damn all these good tips for doing quick percentage math @.@
This is true for quite a few other things as well. For instance, “hertz” really just means “per second”. However, it’s easier to write/type “Hz” instead of “s^-1”.
This isn’t the only property that makes mental math easy. What is 3.5% of 62? Inverting the percentages doesn’t really help but if we realized 3.5% = 1%+1%+1%+0.5% we have some easy math. This gives us 0.62+0.62+0.62+0.62/2=2.17
Also remember there isn’t a teacher sitting over your shoulder to check your work, most of the time you do mental math, getting close is good enough. 62 is close to 60 and 3.5% is close to 4, and 60 x 4% is easy math to get an answer that will be a little less than 2.4.
Okay this is something everyone should know. Great tip!
thanks!
Proof:
x/100 * y = xy/100 = y/100 * x
Here’s the technical mathematical explanation:
Yuh-doy!
(jk, thanks for actually posting proof)
Damn… You just blew my mind O_O
I aim to please ;)
This is actually so useful, thanks! I feel I used head maths less and less (I’m an engineer and rarely need to do a calculation without a calculator) but this will be an excellent sanity check, or a quick calc when out and about with prices or something.
Many thanks for pointing out something that ought to be obvious haha
It wasn’t obvious until I saw this also. No judgement!
Man, if I heard or figured this out before the GRE, I would have aced that math portion.
I could’ve used this while I was still in school. Sad!
Transitive property of multiplication is crazy, dawg
I don’t see how that explanation makes sense. It’s not showing the same thing.
The example in the title is saying .50 times 7 is equal to .07 times 50 which is not just changing the order like in your explanation.
They’re leaving out one step.
.50 * 7 = 50 * 1/100 * 7 = 50 * 7/100
.
,
Explanation: Commutative Property. So, 0.07 times 50 will always equal 50 times 0.07
Slightly better explanation: 7 × 1/100 × 50 will always equal 50 × 1/100 × 7 (the percentage sign (%) stands for /100)
