Two pipes A and B can fill a tank in 20 and 30 minutes respectively. If both pipes are opened simultaneously, how long will it take to fill the tank?
Answer: B
Part of the tank filled by A in 1 min = 1/20. Part filled by B in 1 min = 1/30. Part filled by both in 1 min = \(1/20 + 1/30 = (3+2)/60 = 5/60 = 1/12\). So, both pipes together will fill the tank in 12 minutes.