In the bottom of a water tank, there are two drains A and B. If only A is open, it takes 30 minutes to empty a full tank and if only B is open, it takes 20 minutes. If for 10 minutes both drains are open, then B is closed, how much time it takes to empty a full tank?
1
18 minutes
2
15 minutes
3
17 minutes
4
20 minutes
5
None of the above