YSK - X percent of Y is equal to Y percent of X. So, if you want to find out what 7% of 50 is, you could instead find out what 50% of 7 is, which is 3.5. This means that 7% of 50 is also equal to 3.5.

Percentage is just a fancy form of decimal point. I just multiply them together then move decimal digit to the left twice.

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.

Proof:

(x/100) * y = xy/100 = (y/100) * x

Okay this is something everyone should know. Great tip!

Damn... You just blew my mind O_O

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

I could've used this while I was still in school. Sad!

Man, if I heard or figured this out before the GRE, I would have aced that math portion.

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.

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)

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