Pipe 'A' can fill a tank in 18 hours and pipe 'B' can fill in 30 hours. Both the pipes opened together but after 5 hours pipe 'B' closed. In how many hours will pipe 'A' take to fill the remaining part of tank.

Pipe A fills 1/18 of tank per hour; Pipe B fills 1/30 per hour. Combined rate = 1/18 + 1/30 = 5/90 + 3/90 = 8/90 per hour. In 5 hours together: 5 × 8/90 = 40/90 = 4/9 filled. Remaining tank = 1 - 4/9 = 5/9. Pipe A alone fills at 1/18 per hour. Time needed = (5/9) ÷ (1/18) = (5/9) × 18 = 5 × 2 = 10 hours. Option C is correct.