A and B can do a piece of work in 21 and 24 days respectively. They start the work together and after some days, A leaves and B completes the rest of the task in 9 days. After how many days did A leave ?

A in a day = 1/21 Part of work completed by B in a day = 1/24 Part of work completed by them together in a day = (1/21)+(1/24)
= 5/56 Part of work completed by B in 9 days
= 9/24 = 3/8 If A left after x days,
(5x/56)+(3/8) = 1
x = 7