Pipe P can fill an empty tank in 15 h and pipe Q can fill the same tank in 45 h. Both pipes open alternatively for 1 h each without any time gap. In how many hours 90% of the tank will be filled?
1
\(20\frac{1}{6}\;h\)
2
\(24\frac{1}{7}\;h\)
3
\(21\frac{1}{6}\;h\)
4
\(20\frac{1}{4}\;h\)
5
\(30\frac{1}{4}\;h\)