A and B can do a work in 8 days, B and C can do the same work in 12 days, and A, B, and C can do it in 6 days. In how many days can A and C do it?
Answer: C
One day's work: (A+B)=1/8, (B+C)=1/12, (A+B+C)=1/6. A's work = (A+B+C) - (B+C) = 1/6 - 1/12 = 1/12. C's work = (A+B+C) - (A+B) = 1/6 - 1/8 = (4-3)/24 = 1/24. (A+C)'s one day work = 1/12 + 1/24 = (2+1)/24 = 3/24 = 1/8. So, A and C can do the work in 8 days.