A can do a piece of work in 80 days. He works at it For 10 days and then B alone finishes the work in 42 days. The two together could complete the work in?

A's 10 day's work = (10 x 1/80) = 1/8
Remaining work = (1 - 1/8) = 7/8
∵ 7/8 work is done by B in 42 days
⇒ Whole work will be done by B in (42 x 8/7) = i .e., 48 days.
∴ (A + B)'s day's work = (1/80 + 1/48) = 8/240 = 1/30
Hence, A and B together can finish it in 30 days.