A can finish a work in 21 days and B can do the same work in 15 days. B worked for 10 days and left the job. In how many days, A alone can finish the remaining work?  Pick one:  A) 6  B) 5  C) 5.5  D) 7  E) 8
B's 10 day's work = ( 1 x 10 ) = 2 .   15 3   Remaining work = ( 1 - 2 ) = 1 .   3 3   Now, 1 work is done by A in 1 day.   21   Therefore 1 work is done by A in ( 21 x 1 ) = 7 days.   7
<result>D</result>
A can finish a work in 21 days and B can do the same work in 15 days. B worked for 10 days and left the job. In how many days, A alone can finish the remaining work?