If q varies directly as the square of r and r decreases by 20%.find the percentage change in q

Let q = kr^2

When r decreases by 20%, r = 0.8r

Therefore, q = k(0.8r)^2 = 0.64kr^2

The percentage change in q = (0.64kr^2 - kr^2)/kr^2 * 100
= -36%