Two pipes X and Y can fill a tank in 10 hr and 12 hr respectively. While pipe Z can empty the tank in 20 hr. All three pipes are opened together for first 6 hr, then pipe Z is closed. In how much time from the start will the tank be full?
1
6.11 hr
2
7.09 hr
3
5.09 hr
4
4.11 hr
5
None of these.