as a matter of fractions, 1 whole is hmm usually same/same.
but for this specific case, is 8/8, 8/8 is 1/1 or just 1 whole.
so the whole amount of books Tony has is 8/8 then.
we know that 1/8 of that is mystery books, we also know that is really 3 books, so 1/8 is 3 books in his collection, how many are there in all?
[tex]\bf \begin{array}{ccll}\stackrel{
whole}{fraction}&amount\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
\frac{1}{8}&3\\\\
\frac{8}{8}&x
\end{array}\implies \cfrac{\quad\frac{1}{8} \quad }{\frac{8}{8}}=\cfrac{3}{x}\implies \cfrac{1}{8}\cdot \cfrac{8}{8}=\cfrac{3}{x}
\\\\\\
\cfrac{1}{8}=\cfrac{3}{x}\implies x=\cfrac{8\cdot 3}{1}[/tex]
