A can do a work in 21 days and B alone can do it 28 days, they begin the work together, but A left after some days. B completed the remaining work in 21 days. After how many days did A leave ?  Choose the correct choice from the following answers:
A) 10
B) 8
C) 3
D) 15
E) 16
Explanation:   (A+B) 1 day work = {1/21+1/28}=1/12 = 12days is required for A&amp;B   B’S 1 day work = 1/28   B’s 21 days work = 21/28   1 - 21/28 = 7/28 - 1/4   1/4 *12= 
<gadget id="calculator">1/4 *12</gadget>
<output>3</output>
3 days   Answer: Option 3
<result>C</result>
A can do a work in 21 days and B alone can do it 28 days, they begin the work together, but A left after some days. B completed the remaining work in 21 days. After how many days did A leave ?