A circle has diameter 20. What is the length of an arc with a central angle of 72°?

The diameter is $20$, so the radius is $10$ and the circumference is $2\pi(10)=20\pi$. The arc is $\frac{72}{360}=\frac{1}{5}$ of the circle, so its length is $\frac{1}{5}\cdot20\pi=4\pi$, choice A. Choice C ($10\pi$) is half the circumference, the arc for $180^\circ$. Choice B ($8\pi$) is $\frac{2}{5}$ of the circumference, for $144^\circ$. Choice D ($14.4\pi$) uses $72\%$ of the circumference rather than $\frac{72}{360}$. Use the radius in $2\pi r$, not the diameter, to find the circumference.