Question

The percent of change of 80 books to 64 books

Answer 1

(80-64/80)x  100= (16/80) x 100 =20%

Answer 2

So from 80 to 64, the change is the difference or 16 books.

now, if we take 80 to be the 100%, how much is 16 off of it in percentage?

$\bf \begin{array}{ccll} amount&\%\\ \text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\ 80&100\\ 16&p \end{array}\implies \cfrac{80}{16}=\cfrac{100}{p}\implies p=\cfrac{16\cdot 100}{80}$