How Do You Practically Calculate Common Percentages?

Question

I was wondering if there is any quick way of calculating some common percentages such as $15\%$, $25\%$, $50\%$, $75\%$, $90\%$ etc.

The best think I could come up with is just simplifying the fraction they represent. So for $75%$, which is $\frac{3}{4}$, you just multiply by $3$ and divide by $4$. Is this the method some advanced mathematicians also use for quick, mental calculations? Or is there a better way? Thank you in advance.

Answer 1

10% is easy, just divide by 10, in other words shift the decimal point. Half of the result gives you 5%, add them and that's 15%.

50%, just divide by 2.

25%, divide by 4 either in one step, or by 2 then by 2 again.

75%, divide by 2 to get 50%, divide by 2 again to get 25%, add those two results.

90%, easy to find 10%, then subtract from the original.

Answer 2

You can just multiply using decimals. For example, if you want to find the $25\%$ of 70, you can just multiply $$70 \cdot 0,25 = 17,5$$ In general, to find the $a \%$ of $x$ just do $$x \cdot 0,a$$