A completes a work in 15 days. B complete the same work in 20 days. A started working alone after 1 days B joined him. How many day will they now take together to complete the remaining work?

Work of A for 1 day = 1/15
Work of B for 1 day = 1/20
Work of (A + B) together for 1 day = 1/15 + 1/20 = (4 + 3)/60 = 7/60
Remaining work after A alone does for 1 day = 1 - 1/15 = 14/15
∵ 7/60 part-work can be complete by (A + B) in 1 days
∴ 14/15 part-work can be completed by (A + B) in = (60/7) x (14/15) = 8 days.