let's say there are "x" amount of questions in the test, thus "x" is the 100%.
we know that 18 is the 75%, what is "x" anyway?
[tex]\bf \begin{array}{ccll}
amount&\%\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
x&100\\
18&75
\end{array}\implies \cfrac{x}{18}=\cfrac{100}{75}\implies x=\cfrac{18\cdot 100}{75}[/tex]
