Question

The cost of pizza varies directly as the square of its radius. If a pizza with radius 6 inches costs 9 $\$$, then the cost of a 12 inch pizza is

• A. $\$30$
• B. $\$27$
• C. $\$36$

Answer 1

The correct option is C $\$36$

Let the cost of pizza be $C$ and radius be $r$
Given $C\propto r^2$
$\Rightarrow C=k\times r^2$
A pizza with a radius of $6$ inches costs $9\$$
$\Rightarrow 9=k\times 6^2$
$\Rightarrow k=\frac{1}{4}$

$\therefore C=\frac{1}{4}\times r^2$

Let's calculate cost of pizza with $r=12$ inches

$C=\frac{1}{4}\times {12}^2=36\$$