If each side of a square is increased by 40%, by what percentage does its area increase?

Area varies as the square of the side, so the new area is $1.4^2=1.96$ times the old — a rise of 0.96, i.e. 96%. By the successive-change rule the increase is $40+40+(40\times 40)/100=96$%. Both partial answers are offered as traps: 80% stops at 40 + 40, and 16% keeps only the product term $(40\times 40)/100$. 40% assumes area scales the way a length does.