Discussion

Explanation:

Let the time taken by each of them be X, Y and Z respectively.

∴ $\frac{1}{X}+\frac{1}{Y}+\frac{1}{Z}=\frac{1}{20}$    …(1)

Also, Y does half the work done by X and Z in a day, hence

$\frac{1}{Y}=\frac{1}{2}\left(\frac{1}{X}+\frac{1}{Z}\right)$ or

$\frac{1}{X}+\frac{1}{Z}=\frac{2}{Y}$    …(2)

From (1) and (2)

$\frac{2}{Y}+\frac{1}{Y}=\frac{1}{20}$ 

Y = 60 days

Hence, option (c).

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