13) In a rectangular tank with a square base of side $40 \mathrm{~cm}$, water is added until it reaches a level of $30 \mathrm{~cm}$. Then, a solid cube with a side of $20 \mathrm{~cm}$ is completely submerged in the tank. What is the new water level?
(A) $32 \mathrm{~cm}$
(B) $32.5 \mathrm{~cm}$
(C) $35 \mathrm{~cm}$
(D) $40 \mathrm{~cm}$
(E) $50 \mathrm{~cm}$
13) The answer is (C). Indicating the rise in water level with $h$, we have $20 \mathrm{~cm} \times 20 \mathrm{~cm} \times$ $20 \mathrm{~cm}=40 \mathrm{~cm} \times 40 \mathrm{~cm} \times h$, from which $h=5 \mathrm{~cm}$, and the level will therefore be $(30+5) \mathrm{~cm}=35 \mathrm{~cm}$.