UPSC CAPF Assistant Commandant Model Paper 4

Given:
Two pipes A and B can fill a tank in 20 and 24 minutes respectively and a waste pipe can empty 2 gallons of liquid per minute.
All three pipes working together fill pipe in 20 minutes
Formula used:
Capacity = Time × Efficiency
Calculation:
∴ Let total capacity = LCM[20, 24, 20] = 120 units
⇒ E(A) = 120/20 = 6, E(B) = 120/24 = 5 and E (A + B + C) = 120/20 = 6 [∵ Capacity = Time × Efficiency]
⇒ E(C) = 6 - 6 - 5 = - 5 units [negative as C is a waste pipe]
∴ C empties 5 units every minute
According to question, 5 units = 2 gallons.
Thus, Capacity of tank = (120/5) × 2 = 48 gallons