A can do a piece of work in 4 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?
Answer: C
Rates per hour: A=1/4, B+C=1/3, A+C=1/2. From A+C=1/2, we get C's rate = 1/2 - A's rate = 1/2 - 1/4 = 1/4. From B+C=1/3, we get B's rate = 1/3 - C's rate = 1/3 - 1/4 = (4-3)/12 = 1/12. So, B alone will take 12 hours.