A and B together can do a work in 20 days. B and C together can do the same work in 25 days. C and A together can do the same work in 15 days. A alone can do the same work in a day?

Given rates: A+B = 1/20, B+C = 1/25, C+A = 1/15 work per day. Adding all three equations: 2(A+B+C) = 1/20 + 1/25 + 1/15 = (15 + 12 + 20)/300 = 47/300. Therefore A+B+C = 47/600. To find A alone: A = (A+B+C) - (B+C) = 47/600 - 1/25 = 47/600 - 24/600 = 23/600 work per day. Thus A alone takes 600/23 days to complete the work. This is a standard combined work problem requiring algebraic manipulation of rates.