Answer
The cross-sectional area of the artery increases by a factor of 4.0
Work Step by Step
The original area of the artery is $A_0 = \pi r^2$.
The new radius is $2.0~r$. We can find the ratio of the new area $A$ to the original area $A_0$.
$ratio = \frac{A}{A_0} = \frac{\pi (2.0r)^2}{\pi r^2} = \frac{4.0 ~\pi r^2}{\pi r^2} = 4.0$
The cross-sectional area of the artery increases by a factor of 4.0.
