A can do a piece of work in 5 hours; B and C together can do it in 3 hours, while A and C together can do it in 2 hours. How long will B alone take to do it?	Choose the correct choice from the following choices:  A) 30  B) 20  C) 10  D) 6  E) 5

Rationale: "a ' s 1 hour ' s work = 1 / 5 ; ( b + c ) ' s 1 hour ' s work = 1 / 3 ; ( a + c ) ' s 1 hour ' s work = 1 / 2 . ( a + b + c ) ' s 1 hour ' s work = ( 1 / 5 + 1 / 3 ) = 8 / 15 . b ' s 1 hour ' s work = ( 8 / 15 - 1 / 2 ) = 1 / 30 . therefore a alone will take 30 hours to do the work . a"
Answer: A