Q119
ArithmeticTime & Work, Pipes & Cisterns
A and B can do a work in 12 days, B and C in 15 days, C and A in 20 days. If A, B and C work together, they will complete the work in
- a.5 days
- b.10 days
- c.12 days
- d.18 days
Answer: (B) 10 days
Adding the three pairs counts each person twice: $2(A + B + C) = \frac{1}{12} + \frac{1}{15} + \frac{1}{20} = \frac{1}{5}$. So A + B + C = $\frac{1}{10}$ per day, i.e. 10 days. 5 days forgets to halve the sum.