Question: a can do a certain job in 12 days?
Answer: of days as A takes to do 100% of work. B is 60% more efficient than A, means that the rate of B is 1.6 times as greater as that of A. So, if A can complete the job in 12 days, then B complete the same job in 12/1.6 = 12/(8/5) = 15/2 = 7.5 days.