Mangala completes a piece of work in 9 days, Raju completes the same work in 21 days. If both of them work together, then the number of days required to complete the work is? Pick
A) 8 days	B) 6 days	C) 11 days	D) 12 days	E) 14 days
If A can complete a work in x days and B can complete the same work in y days, then, both   of them together can complete the work in x y/ x+ y days.   That is, the required No. of days = 9× 21/30 = 6 days   6 days)
<result>B</result>
Mangala completes a piece of work in 9 days, Raju completes the same work in 21 days. If both of them work together, then the number of days required to complete the work is?