A water tank can be filled by two pipes, Pipe A and Pipe B, in 20 minutes and 30 minutes respectively. How long will it take to fill the tank if both pipes are opened at the same time?
Correct: A
The rate of Pipe A filling the tank is 1 tank / 20 minutes, and the rate of Pipe B is 1 tank / 30 minutes. Combined, their rates are (1/20 + 1/30) tanks per minute. Finding a common denominator, we have (3/60 + 2/60) = 5/60 = 1/12 tanks per minute. Therefore, it will take 12 minutes to fill the tank.