A and B can do a piece of work in 21 and 24 days respectively. They started 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?	Pick one:
A) 5 days	B) 6 days	C) 7 days	D) 8 days	E) 9 days
In 9 days B completes 9/24 i.e., 3/8th of the work.   Hence 1 - 3/8 = 5/8th of the work is completed by A and B together.   i.e., x (1/21 + 1/24) = 5/8 =&gt; x = 7 days.   ANSWER:7 days
<result>C</result>
A and B can do a piece of work in 21 and 24 days respectively. They started 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?