A can do a piece of work in 5 days. B can do it in 20 days. With the assistance of C, they completed the work in 2 days. C alone can do it in ______________days.

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  1. 4
  2. 5
  3. 6
  4. 3

Answer (Detailed Solution Below)

Option 1 : 4
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Detailed Solution

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Given:

A can complete the work in 5 days.

B can complete the work in 20 days.

With the assistance of C, they complete the work in 2 days.

Formula used:

Work done per day = Efficiency

Calculation:

LCM of (5, 20, 2) = 20 (Total work units)

Efficiency of A = \( \frac{20}{5} = 4 \) units/day

Efficiency of B = \( \frac{20}{20} = 1 \) unit/day

Efficiency of (A + B + C) = \( \frac{20}{2} = 10 \) units/day

⇒  4 + 1 + C = 10  

⇒  C = 10 - 5 

⇒  C = 5 units/day

Time taken by C alone = \( \frac{20}{5} = 4 \) days

∴ C alone can complete the work in 4 days.

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