Pipes A nd B can fill a tank in 18 hours and 27 hours, respectively. If both the pipes are opened on alternate hours, one at a time, and pipe A is opened for the first hour, then in how many hours will the tank be filled completely?
1
21
2
\(20 \frac{1}{2}\)
3
20
4
\(21 \frac{1}{2}\)