Two pipes A and C together can fill a tank in 12 hours. Ratio of efficiency C and A is 3 : 4. They together filled the tank for the first 7 hours. Then A and C are closed and another pipe B is opened. Now if tank is filled in another 17.5 hours then B alone can fill the tank in how many hours ?
1
28 hours
2
21 hours
3
42 hours
4
35 hours
5
32 hours