A and B can do a piece of work in 72 days; B and C can do it in 120 days; A and C can do it in 90 days. In what time can A alone do it?
Q1. A and B can do a piece of work in 72 days; B and C can do it in 120 days; A and C can do it in 90 days. In what time can A alone do it?
Answer: 120 days
Explanation: Adding all three pair-rates gives 2(1/A+1/B+1/C) = 1/30; subtracting B+C's rate (1/120) from 1/60 gives 1/A = 1/120, so A = 120 days.