Question

If $r$ varies directly as the cube of $s$ and $r=5$ when $s=3$, find $r$ when $s=2$

• A. $135$
• B. $135/8$
• C. $35/8$
• D. None of these

Answer 1

The correct option is B $135/8$

Given:- r =$5$, s= $3$
Formula - $r=\frac{k}{s^3}$ .....step $\left(1\right)$
$5=\frac{k}{3^3}$
$5=\frac{k}{27}$ = $135$ ....... step $\left(2\right)$
Rewrite the equation from step 1 substituting in the value of k found in step 2.
$r=\frac{135}{2^3}$
$r=\frac{135}{8}$