A and B can do a piece of work in 12 days. B and C in 15 days. C and A in 20 days. A alone can do the work in?

∵ [(A + B) + (B + C) + (C + A)]'s 1 day's work = (1/12 + 1/15 + 1/20) = 1/5
⇒ 2(A + B + C)'s 1 days work = 1/5
⇒ (A + B + C)'s 1 day's work = 1/10
&rArr A's 1 day's work = (1/10 - 1/15) = 1/30
∴ A alone can finish it in 30 days.