Math Help - area and cost

1. area and cost

An aluminum plate is made according to the following dimensions. THe outer radius is 12 inches and the inner radius is 5 insches. Use Pi = 3.14 as an approximate value. (a) Hamy square inches in area is the aluminum plate? (b) If the cost to construct the plate is 5.50 per square inch, what is the construction cost?

2. I'm going to assume that this plate is donut-shaped. Let $R = 12\,in$ and $r = 5\,in$. The area of this plate would be
\begin{aligned}
A &= \pi R^2 - \pi r^2 \\
&= 12^2 \pi - 5^2 \pi \\
&= 144 \pi - 25 \pi \\
&= 119 \pi \\
&\approx 373.85\,in^2 \\
\end{aligned}

Multiply this amount by \$5.50/sq. in. to get the cost.