Question

If x varies directly as y, and x = 7.5 when y = 10, find x when y = 4

Answer 1

$\bf \qquad \qquad \textit{direct proportional variation} \\\\ \textit{\underline{y} varies directly with \underline{x}}\qquad \qquad y=kx\impliedby \begin{array}{llll} k=constant\ of\\ \qquad variation \end{array} \\\\[-0.35em] \rule{34em}{0.25pt}$

$\bf \stackrel{\textit{"x" varies with "y"}}{x=ky}\qquad \textit{we also know that } \begin{cases} x=7.5\\ y=10 \end{cases}\implies 7.5=k(10) \\\\\\ \cfrac{7.5}{10}\implies \cfrac{~~\frac{15}{2}~~}{10}=k\implies \cfrac{3}{4}=k~\hfill \boxed{x=\cfrac{3}{4}y} \\\\\\ \textit{when y = 4, what is \underline{x}?}\qquad x=\cfrac{3}{~~\begin{matrix} 4 \\[-0.7em]\cline{1-1}\\[-5pt]\end{matrix}~~}(~~\begin{matrix} 4 \\[-0.7em]\cline{1-1}\\[-5pt]\end{matrix}~~)\implies x=3$