Question

75% of ____ apples is 15 apples?

Answer 1

75% of "x" is 15, meaning that "x" is the 100% then.

so, since 15 is the 75%, what is the 100% or "x"?

$\bf \begin{array}{ccll} amount&\%\\ \cline{1-2} 15&75\\ x&100 \end{array}\implies \cfrac{15}{x}=\cfrac{75}{100}\implies \cfrac{15}{x}=\cfrac{3}{4} \\\\\\ 60=3x\implies \cfrac{60}{3}=x\implies 20=x$