let's say that number is "x", therefore "x" is the 100%, and we know that 15 is 1 and 1/2% of that,
[tex]\bf \begin{array}{ccll}
amount&\%\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
x&100\\
15&1\frac{1}{2}
\end{array}\implies \cfrac{x}{15}=\cfrac{100}{1\frac{1}{2}}\implies \cfrac{x}{5}=\cfrac{100}{\frac{3}{2}}\implies \cfrac{x}{5}=\cfrac{\frac{100}{1}}{\frac{3}{2}}
\\\\\\
\cfrac{x}{5}=\cfrac{100}{1}\cdot \cfrac{2}{3}\implies \cfrac{x}{5}=\cfrac{200}{3}[/tex]