There are three pipes, A, B and C, attached to container. A and B can fill the container alone in 20 and 30 mins, respectively whereas C can empty the container alone in 45 mins. The three pipes are kept opened alone for one minute each in the the order A, B and C. The same order is followed subsequently. In how many minutes will the reservoir be full?

In each 3-minute cycle, A fills 1/20, B fills 1/30, and C empties 1/45 of the container. Net fill per cycle = 1/20 + 1/30 - 1/45 = 11/180. After 16 cycles (48 minutes), 176/180 = 44/45 is filled. In the 17th cycle, A fills 1/20, which would overflow (44/45 + 1/20 > 1). So we track minute-by-minute: after 47 minutes, the cumulative fill is 179/180. The next minute (A) would complete the filling, taking us to 179/180 + 1/20 = 185/180 > 1. Therefore, the reservoir fills exactly at 47 minutes. Option D is correct.