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1. A, B and C can do a piece of work in 20, 30 and 60 days respectively. In how many days can A do the work if he is assisted by B and C on every third day?

A. 12 days

B. 15 days

C. 16 days

D. 18 days

Solution:

\begin{aligned} A's 2 day's work = \frac{1}{20} \times 2 = \frac{1}{10} \\ (A + B + C)'s 1 day's work \\ = \frac{1}{20} + \frac{1}{30} + \frac{1}{60} = \frac{6}{60} = \frac{1}{10} \\ Work done in 3 days = \frac{1}{10} + \frac{1}{10} = \frac{1}{5} \\ Now, \frac{1}{5} work is done in 3 days \\ ∴ Whole work will be done in \\ 3 \times 5 = 15 days \end{aligned}

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