Three pipes X, Y and Z can fill a pool in 12, 15 and 20 hours respectively. If pipe X is open all the time and pipe Y and pipe Z are open for 1 hour each alternatively, what will be the total hours required to full the pool?
1
6 hours
2
\(6\frac{1}{2}\) hours
3
7 hours
4
\(7\frac{1}{2}\) hours