[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{34em}{0.25pt}\\\\ \stackrel{\textit{\underline{c}cost directly proportional to \underline{p}izzas}}{c=kp} \\\\\\ \textit{we know that } \begin{cases} p=2\\ c=25 \end{cases}\implies 25=k(2)\implies \cfrac{25}{2}=k\implies 12\frac{1}{2}=k[/tex]
