Assuming that the reservoir can be drained at a constant rate, the time will be calculated as follows:

In one hour,

1 / 12 = 0,083 or 8,3% of the reservoir will be drained through the large pipes

1 / 18 = 0,055 or 5,5% of the reservoir will be drained through the small pipes

Therefore,

In one hour , 8,3 + 5,5 = 13,8 % of the reservoir will be drained by large, small pipes together

Then,

The reservoir will be drained in 100 / 13,8 = 7,25 hours