Explanation:

A does 3/5th of work in 15 days.
Time Taken by A to complete the work
= 5/3 x 15 = 25 days
Efficiency of B is 25% more than that of A.
If Work done by A in one day = 4, then work done by B in one day = 5
Ratio of time taken by A and B = 5 : 4
Time taken by B to complete the work
= 4/5 x 25 = 20 days
A and B worked for five days.
Total work Completed by A and B in 5 days
= 5 (1/25 + 1/20)
= 1/5 + 1/4 = 9/20
Work Left = 11/20 which is done by C in 11 days.
Time taken by C to complete the whole work = 20/11 x 11
C = 20 days
Ratio of Efficiency of A and C = 20/25
Efficiency of C is 25 % more than that of A.