To do a certain work, the ratio of the efficiencies of A, B and C is 3 : 5 : 6. Working together, they can complete the same work in 15 days. A and C worked together for 7 days. B alone can complete the remaining work in:
1
30 days
2
28 days
3
\(27{ {3} \over 5}\) days
4
\(29{ {2} \over 5}\) days