\(Method\ \#1\\\\\begin{array}{ccc}31&-&40\%\\x&-&100\%\end{array}\ \ \ \ |cross\ multiply\\\\\\40x=31\cdot100\ \ \ \ |divide\ both\ sides\ by\ 40\\\\x=\frac{3100}{40}\\\\\boxed{x=77.5}\)
\(Method\ \#2\\\\p\%=\frac{p}{100}\to40\%=\frac{40}{100}=0.40=0.4\\\\x-the\ number\\\\40\%\ of\ the\ number\ is\ 31:\\\\0.4x=31\ \ \ \ |divide\ both\ sides\ by\ 0.4\\\boxed{x=77.5}\)