a can do.a piece of work in 6 days and B in 8 days each working 7hours a day In what time will they finish it together, working 8hrs a day

A can complete the work in 7*6 hrs = 42 hrs. A can complete in 1 hr = 1/42 part. B can complete the work in 7*8 hr = 56 hrs. A&B together do in 1 hr = 1/42 + 1/56 = 7/168 = 1/24 part. So they will do in 8 hrs = 8*1/24 = 1/3 part. Hence they need 1÷1/3 = 3 days to complete the work.

The first question's answer is D

and the second question's answer is A