The ratio of working efficiency of A and B is 4 : 5. If A alone completes the work in 15 days then in how many days can B alone complete the work?

12 days

We have,

The ratio of working efficiency of A and B is 4 : 5. If A alone completes the work in 15 days.

We know that, if the work is same then the efficiency is directly proportional to the time.

So, The ratio of working efficiency of A and B is 4 : 5.

The ratio of time of A and B is 5 : 4.

So if,

5x = 15

then 4x = 12