The radius of the base of a right circular cylinder is increased by 20%. By what per cent should its height be reduced so that its volume remains the same as before?

Solution (By Examveda Team)

$$\eqalign{ & 20\% \to \frac{{ + 1}}{5} \cr & V = \pi {R^2}h \cr & R \to 5:6 \cr & {R^2} \to 25:36 \cr & \boxed{{R^2} \propto \frac{1}{h}}{\text{ If volume same}} \cr} $$

$$\frac{{11}}{{36}} \times 100 = 30\frac{5}{9}\% $$