A can do a piece of work in 3 hours; B and C together can do it in 6 hours, while A and C together can do it in 9 hours. How long will B alone take to do it? Answers:
A) 4 9/7 hours
B) 3 5/7 hours
C) 2 4/7 hours
D) 3 8/7 hours
E) none of the above
A's 1 hour's work = 1/3 ;   (B + C)'s 1 hour's work = 1/6 ;   (B + C)'s 1 hour's work = 1/9   (A + B + C)'s 1 hour's work = (1/3 + 1/6) = 1/2   B's 1 hour's work = (1/2 - 1/9) = 7/18   B alone will take 18/7 hours to do the work.   =2 4/7   Answer = 2 4/7 hours
<result>C</result>
A can do a piece of work in 3 hours; B and C together can do it in 6 hours, while A and C together can do it in 9 hours. How long will B alone take to do it?