A can finish a work in 15 days, B can finish the same work in 25 days. They work together for 5 days. The rest of the work is finished by A and C in 4 days. Then C alone can finish the work in:

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SSC CHSL Exam 2023 Tier-I Official Paper (Held On: 02 Aug, 2023 Shift 1)
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  1. 18 days
  2. 24 days
  3. 20 days
  4. 21 days

Answer (Detailed Solution Below)

Option 3 : 20 days
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SSC CHSL Exam 2023 Tier-I Official Paper (Held On: 02 Aug 2023 Shift 1)
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100 Questions 200 Marks 60 Mins

Detailed Solution

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Given:

A can finish in 15 days, B can finish it in 25 days.

They work together for 5 days.

Concept used:

Efficiency = (Total work / Total time taken)

Efficiency = work done in a single day 

Calculation:

Let total work be 75 units ( LCM of 15 and 25 is 75)

The efficiency of A

 75 /15 = 5 units

The efficiency of B  

 75 / 25 = 3 units

The efficiency of A+B,

⇒ (5 + 3) units = 8 units

In 5 days total work done is 8 × 5 = 40 units

Remaining work 75 - 40 = 35 units

In the last 4 days, A does 4 × 5 = 20 units

Remaining work 35 - 20 = 15 units done by C in 4 days

So C does 75 units in (75 / 15) × 4 = 20 days

∴ The correct option is 3

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