A can do a piece of work in 4 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. 7
  4. 6

Answer (Detailed Solution Below)

Option 2 : 5
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Detailed Solution

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Given:

A can do a piece of work in 4 days.

B can do a piece of work in 20 days.

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

Formula used:

Work done by A in 1 day = 1/4

Work done by B in 1 day = 1/20

Work done by A, B, and C together in 1 day = 1/2

Calculations:

Work done by A, B, and C together in 1 day = Work done by A in 1 day + Work done by B in 1 day + Work done by C in 1 day

⇒ 1/2 = 1/4 + 1/20 + 1/C

⇒ 1/C = 1/2 - 1/4 - 1/20

⇒ 1/C = (10 - 5 - 1) / 20

⇒ 1/C = 4/20

⇒ C = 5 days

∴ The correct answer is option 2.

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