Two pipes P and Q, when opened alone can fill the tank in 20 and 30 hours respectively. If both pipes are opened together, then in how many hours will the tank be filled?

Question: Two pipes P and Q, when opened alone can fill the tank in 20 and 30 hours respectively. If both pipes are opened together, then in how many hours will the tank be filled?

Solution:
Part of tank filled by pipe P in 1 hour = 1/20
Part of tank filled by pipe Q in 1 hour = 1/30

Tank filled by both pipes in 1 hour = 1/20 + 1/30
= 5/60
= 1/12

∴ Complete tank will be filled by both in 1/1/12 hours
= 12 hours