[tex] \bf \qquad \qquad \textit{direct proportional variation}\\\\\textit{\underline{y} varies directly with \underline{x}}\qquad \qquad y=kx\impliedby \begin{array}{llll}k=constant\ of\\\qquad variation\end{array}\\\\[-0.35em]\rule{31em}{0.25pt}\\\\\stackrel{\textit{for every \boxed{s}aved 5, his \boxed{p}arents give 10}}{p=ks}\\\\\\\textit{we also know that }\begin{cases}s=5\\p=10\end{cases}\implies 10=k5\implies \cfrac{10}{5}=k\implies 2=k [/tex]
