Question

By what percent must we decrease the number A to obtain the number 4A/5?

Answer 1

$\bf \cfrac{4A}{5}\implies \cfrac{4}{5}\cdot A\impliedby \textit{A is being reduced to four fifths} \\\\\\ \stackrel{\textit{let's convert four fifths to percentage}}{\cfrac{4}{5}\cdot 100\implies \cfrac{400}{5}\implies 80}$

so 4/5 of A is really 80% of A then.

since a whole A is 100% of A, 100% - 80% = 20%, A is being reduced by 20%, namely 1/5.