Pipes A and B can fill a tank in 30 and 20 minutes, respectively. There is also an emptying pipe C attached to the tank. If all the three pipes are opened together, the tank is filled in 15 minutes. If the tank is one-third empty, then C alone will empty the tank completely in:
Show answer & explanation
Correct answer: C
Explanation
The correct answer is Option 3 i.e. 40 minutes.
LCM of 30, 20, 15 = 60 units (total work)
A’s efficiency = 60/30 = 2 units/min
B’s efficiency = 60/20 = 3 units/min
A + B + C = 60/15 = 4 units/min
So C’s efficiency = 4 − (2 + 3) = −1 unit/min (negative since it empties)
Tank is one-third empty = 60/3 = 20 units
Filled tank = 60 - 20 = 40 units
Time by C alone = 40/1= 40 minutes