Time and work Quantitative aptitude question

[Q] 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) 45 days
(b) 15 days
(c) 30 days
(d) 25 days

Solution

Answer   (b)
Explanation
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Amount of work A can do in 1 day = 1/20

Amount of work B can do in 1 day = 1/30

Amount of work C can do in 1 day = 1/60

A is working alone and every third day B and C is helping him

Work completed in every three days = 2 × (1/20) + (1/20 + 1/30 + 1/60) = 1/5

So work completed in 15 days = 5 × 1/5 = 1

Ie, the work will be done in 15 days