Explanation:

Solution: 
Efficiency (A : B) = 5 : 4
Number of days(A : B) = 4x : 5x = 4x : 25
∴ Number of days required by A to finish the work alone = 4x
= 4 x 5 = 20.
A and B work together for last 5 days = 5 x 9 = 45%
Efficiency of A = 5% and B’s efficiency = 4%
∴ No. of days taken by A to complete 55% work = 55/5 = 11days