well, we know that 13.7 is the whole or 100%, what is 13.1 off of it in percentage?
[tex]\begin{array}{ccll} amount&\%\\ \cline{1-2} 13.7&100\\ 13.1&x \end{array}\implies \cfrac{13.7}{13.1}=\cfrac{100}{x}\implies 13.7x=1310 \\\\\\ x=\cfrac{1310}{13.7}\implies x \approx 95.62[/tex]
