Pipes A and B fill a tank in 12 h and 15 h; pipe C empties it in 6 h. A and B are open for 5 h, then C is also opened. From the moment C opens, how long until the tank is empty?

1. A+B rate = 1/12 + 1/15 = 9/60 = 3/20 per hour; in 5 h they fill 5 × 3/20 = 3/4 of the tank. 2. After C opens, net rate = 3/20 − 1/6 = 9/60 − 10/60 = −1/60 per hour (emptying). 3. Time to drain 3/4 = (3/4) ÷ (1/60) = **45 hours**. _Source: IBPS RRB Officer Scale I Prelims PYQ (Examrace), Q.66_