2015 AMC 12A Problem 7

Let $r,h$ and $R,H$ be the radii and heights of the first and second cylinders. The volumes are equal, so $\pi r^2 h = \pi R^2 H,$ and $R = 1.1r.$ Then $\pi r^2 h = \pi(1.1r)^2 H$ $= \pi(1.21 r^2) H.$ Dividing by $\pi r^2$ yields $h = 1.21H,$ so the first height is $21\%$ more than the second. Thus, the correct answer is D.