Question

If a varies jointly as b and c and inversely as the square of $d$, and $a=120$ when $b=5$, $c=2$, and $d=9$, find $a$ when $b=12$, $c=9$ and $d=9$.

• A. $1256$
• B. $1246$
• C. $1296$
• D. None of these

Answer 1

The correct option is C $1296$

$y=\frac{kxz}{w}\Rightarrow a=\frac{kbc}{d^2}$
$120=\frac{k\left(5\right)\left(2\right)}{9^2}$
$9720=10k$
$k=972$
$a=\frac{972bc}{d^2}$
$a=\frac{972\left(12\right)\left(9\right)}{9^2}$
$a=108\times 12$
$1296$