Question: a can do piece of work in 14 days?
Answer: If A can do the wrok in 14 days, his work per day = 1/14.
if B can do the work in 21 days, his work per day = 1/21.
if A and B work together, work per day = (1/14)+(1/21) = 5/42.
Hence work will be completed, if and B work together = 42/5 = [ 8+(2/5) ] days.