Question

One inlet pipe can fill an empty pool in 12 hours, and a drain can empty the pool in 20 hours. How long will it take the pipe to fill the pool if the drain is left open?

Answer 1

30 hours For this problem, going to assume that the actual flow rate for both pipes is constant for the entire duration of either filling or emptying the pool. The pipe to fill the pool I'll consider to have a value of 1/12 while the drain that empties the pool will have a value of 1/20. With those values, the equation that expresses how many hour it will take to fill the pool while the drain is open becomes: X(1/12 - 1/20) = 1 Now solve for X X(5/60 - 3/60) = 1 X(2/60) = 1 X(1/30) = 1 X/30 = 1 X = 30 To check the answer, let's see how much water would have been added over 30 hours. 30/12 = 2.5 So 2 and a half pools worth of water would have been added. Now how much would be removed? 30/20 = 1.5 And 1 and half pools worth would have been removed. So the amount left in the pool is 2.5 - 1.5 = 1 And that's exactly the amount needed.