Tap A alone fills a tank in 20 hours and tap B alone fills the same tank in 30 hours. A is opened first, and B is opened 5 hours later. Counting from the moment A was opened, the tank is full after how many hours?

In its first 5 hours A alone fills 5/20 = 1/4 of the tank. The remaining 3/4 is filled by both taps at 1/20 + 1/30 = 1/12 of the tank per hour, which takes (3/4) ÷ (1/12) = 9 hours. Total = 5 + 9 = 14 hours. (12 hours is the answer if both taps run from the start, and adding 5 to that 12 double-counts the head start.)