The radius of a cylinder is increased by 20% and its height is decreased by 20%. By what percent does its volume increase?

Volume = π$r^2$h, so the new volume = $(1.2{)}^2\times 0.8=1.44\times 0.8=1.152$ times the old. The increase = 15.2%. (Answering 0 assumes the +20% and the −20% cancel, but the radius is squared and the height is not; $1.2\times 0.8=0.96$ treats the radius as unsquared and gives a 4% fall; 44% counts the radius alone and forgets the shorter height.)