A and B together can do a work in 24 days. B alone does its 1/3 part in 12 days. How long will A alone take to do the remaining work?

Work done by (A + B) in 1 day = 1/24
Work done by B alone in 1 day = 1/(3 x 12) = 1/36
∴ Work of A for 1 day = 1/24 - 1/36 = 1/72
After 12 days the remaining work = 1 - 1/3 = 2/3
∵ 1/72 work is done by A in 1 day
∴ 2/3 work is done by A in (1 x 72)/(1 x 2/3) = 48 days