Question

Suppose y varies directly as x, and y= 3 when x = 7.5. Find y when x = 18.

Answer 1

$y\propto x$

y = k; k is variation constant

we have:

y = 3 when x = 7.5

$\begin{gathered} 3=k\times7.5 \\ \frac{3}{7.5}=\frac{k\times7.5}{7.5} \\ k=\frac{3}{7.5}\text{ or k=}\frac{2}{5}\text{ or k = 0.4} \end{gathered}$

thus the variation equation is

$y=0.4\times x$

then, when x=18

$\begin{gathered} y=0.4\times18 \\ y=7.2 \end{gathered}$