A and B can do a piece of work in 12 days B and C in 15 days C and A in 20 days. In how many days can C alone do it?

Efficiency of (A + B) = 8.33%
Efficiency of (B + C) = 6.66%
Efficiency of (C + A) = 5.00%
∴ Efficiency of A + B + C = (8.33 + 6.66 + 5) / 2 = 10%
∴ Efficiency of C = Efficiency of [(A + B + C) - (A + B) ]
= (10 - 8.33) = 1.66%
∴ No. of days required by C alone = 100/1.66 = 60days