Can you calculate the time that will be taken by 6 large pipes and 6 small pipes together to drain the reservoir?

Question

There is a large fresh water reservoir that has two distinct drainage systems - small pipe and large pipe.

Six large pipes can drain the reservoir completely in exactly 12 hours while six small pipes will take 18 hours to drain the reservoir.

Can you calculate the time that will be taken by 6 large pipes and 6 small pipes together to drain the reservoir?

Answer 1

1/12+1/18=1/x
x=36/5= 7 hrs 12 mins

Answer 2

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