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 the work and B completes the remaining work in 9 days. After how many days did A leave?
Let the total work be LCM of (21, 24) = 168 units.
Amount of work done by A on 1 day =Â $$\ \frac{\ 168}{21}$$ = 8 units
Amount of work done by B on 1 day = $$\ \frac{\ 168}{24}$$ = 7 units
Let p be the number of days for which they worked together.
In P days, the amount of work done by A and B = 15p
Remaining work = 168x-15p, which is completed by B in 9 days.
168x-15p=9*7
p=7
So A left after 7 days.
B is the correct answer.
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