Pipes A and B can fill a tank in 16 hours and 24 hours, respectively, whereas pipe C can empty the full tank in 40 hours. All three pipes are opened together, but pipe A is closed after 10 hours. After how many hours will the remaining part of the tank be filled?

Combined rate when all three open: (1/16 + 1/24 - 1/40) = (15/120 + 5/120 - 3/120) = 17/120 tank/hour. In 10 hours: 10 × 17/120 = 17/12 = 1 5/12 tank filled (tank overflows). But A closes at 10h. After 10h: 17/12 - 10/16 = 17/12 - 5/8 = (34-15)/24 = 19/24 tank filled. Remaining: 5/24. Rate after A closes: (1/24 - 1/40) = (5/120 - 3/120) = 2/120 = 1/60. Time needed: (5/24) ÷ (1/60) = (5/24) × 60 = 12.5 hours. Options A, B, D are incorrect.