two pipes a and b can separately fill a tank in 20 and 15 minutes respectively . a third pipe c can drain off 45 liters of water per minute . if all the pipes are opened , the tank can be filled in 15 minutes . what is the capacity of the tank ?
"1 / 20 + 1 / 15 - 1 / x = 1 / 15 x = 12 20 * 45 = 900 answer : d"
multiply(multiply(inverse(subtract(add(add(divide(const_1, 20), divide(const_1, 15)), divide(const_1, 45)), divide(const_1, 15))), const_3), 15)