Pipes A and B together can fill an empty tank in 30 hours, whereas pipes B and C together can fill it in 24 hours. A, B and C together can fill the tank in 20 hours. In how many hours can pipe B alone fill 35% of the tank?
1
\(10\frac{1}{2}\)
2
14
3
21
4
\(17\frac{1}{2}\)