Two inlet pipes A and B can fill an empty cistern in 22 and 33 hours, respectively. They are switched on together but pipe B had to be closed 7 hours before the cistern was full. How many hours in all did it take the two pipes to fill the cistern?

Pipe A fills at 1/22 per hour, Pipe B at 1/33 per hour. Combined rate is 5/66 per hour. Pipe B operates 7 hours less than Pipe A. If total time is T hours, A works T hours and B works (T-7) hours. Work equation: T/22 + (T-7)/33 = 1. Multiplying by 66: 3T + 2(T-7) = 66, so 5T - 14 = 66, giving 5T = 80 and T = 16 hours. This confirms option D is correct.