According to QUESTION, A and B can do a work in 12 days ∴ (A + B)’s one day’s work = 1 /12 Similarly, (B + C)’s one day’s work = 1/15 and (C + A)’s one day’s work = 1/20 On adding all three,
∴ 2 (A + B + C)’s one day’s work = 1/12+ 1/15 + 1/20 = 1/5 and (A + B + C)’s one day’s work = 1/10 ∴ A, B and C together can complete the work in 10 days.