4629 - Easy math

Fascinating little life hack, for doing percentages:

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x% of y = y% of x

So, for example, if you needed to work out 4% of 75 in your head, just flip it and do 75% of 4, which is easier.

The answer is of course 3, which is so much easier to figure out when you're trying to get 75 percent of 4, rather than 4 percent of 75.

18% of 50 feels hard to calculate. But 50% of 18 is easy, right?" It's 9!
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For all I know these may be the only two examples that work. I didn't experiment at all with this. It's from the internet. It MUST be right.