A and B can do a work in 12 days. B and C in 15 days. C and A in 20 days. How much time will A alone take to do it?
Answer: C
One day's work: A+B=1/12, B+C=1/15, C+A=1/20. Adding these three gives \(2(A+B+C) = 1/12 + 1/15 + 1/20 = (5+4+3)/60 = 12/60 = 1/5\). So, \(A+B+C = 1/10\). To find A's work, we subtract (B+C)'s work: \(A = (A+B+C) - (B+C) = 1/10 - 1/15 = (3-2)/30 = 1/30\). A alone will take 30 days.