Discussion

Explanation:

Let the time taken by them is A, B and C days respectively.

∴ $\frac{1}{A}+\frac{1}{B}+\frac{1}{C}=\frac{1}{20}$   …(1)

$\frac{1}{A}+\frac{1}{C}=\frac{3}{B}$    …(2)

$\frac{1}{\mathrm{A}}+\frac{1}{\mathrm{B}}=\frac{2}{\mathrm{C}}$   …(3)

Adding 1/B both sides in (2) we get,

$\frac{1}{20}=\frac{4}{B}$

⇒ B = 80 days.

Similarly, adding 1/C both sides in (3) we get,

$\frac{1}{20}=\frac{3}{C}$ 

⇒ C = 60 days.

From (1) we get

$\frac{1}{A}+\frac{1}{80}+\frac{1}{60}=\frac{1}{20}$ 

⇒ A = 48 days

Hence, option (b).

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