A tank has two drain pipes. The 1st pipe alone empties the tank in 15 minutes and 2nd pipe alone empties the tank in 10 minutes. If water leaks out at a constant rate, how long (in minutes) does it take to empty the tank if both pipe opened together?

First pipe empties tank in 15 minutes, so its rate is 1/15 tank per minute. Second pipe empties in 10 minutes, so its rate is 1/10 tank per minute. Together, their combined rate is (1/15 + 1/10) = (2/30 + 3/30) = 5/30 = 1/6 tank per minute. Therefore, time to empty the tank together = 1 / (1/6) = 6 minutes. Option A (6) is correct.