Efficiency of A is 20% more than the efficiency of B and the efficiency of C is 40% less than the efficiency of B. Time taken by B alone to complete the work is 30 days. All A , B and C can complete the whole work in x days. If D can complete the whole work in 7x days. Find the time taken by D to complete the work.
1
50 days
2
65 days
3
60 days
4
75 days
5
90 days