A cylinder water tank is 70cm in diameter. To being with, it is fullof water. A leak starts in the bottom so that it loses 10liters of water every hour. How long will it take for the water level to fall by 20cm
the missing water leaves an empty cylinder, of volume
v = πr^2 h = π * 35^2 * 20 = 24500π cm^3
so, dividing by the rate of depletion,
24500π cm^3 / (10000 cm^3/hr) = 2.45π = 7.69 hr