Question

If p varies directly as q and q= 70 when p= 10, find q when p = 12

Answer 1

$\qquad \qquad \textit{direct proportional variation} \\\\ \textit{\underline{y} varies directly with \underline{x}}\qquad \qquad \stackrel{\textit{constant of variation}}{y=\stackrel{\downarrow }{k}x~\hfill } \\\\ \textit{\underline{x} varies directly with }\underline{z^5}\qquad \qquad \stackrel{\textit{constant of variation}}{x=\stackrel{\downarrow }{k}z^5~\hfill } \\\\[-0.35em] ~\dotfill$

$\stackrel{\textit{"p" varies directly as "q"}}{p~~ = ~~kq}\qquad \textit{we also know that} \begin{cases} q=70\\ p=10 \end{cases}\implies 10=k70 \\\\\\ \cfrac{10}{70}=k\implies \cfrac{1}{7}=k\hspace{15em}\boxed{p=\cfrac{1}{7}k} \\\\\\ \textit{when p = 12, what is "q"?}\qquad 12=\cfrac{1}{7}q\implies 12=\cfrac{q}{7}\implies 84=q$