B is 60% more efficient than A, so the ratio of the time it takes B to complete the job to the time it takes A to complete the job is 100% + 60% = 160% : 100%. This can also be written as 8 : 5. Let's suppose it takes B alone x days to complete the job. Then the ratio of the time it takes B to complete the job to the time it takes A to complete the job is 8 : 5, or 8x : 12. Solving for x, we get 8x = 12, so x = 7 1/2 days. This is the number of days it takes B alone to complete the job.