Two persons A and B working separately can sanitise a building in 6 and 10 hours, respectively. They work in stretches of one hour alternately. If A begins at 8 a.m., when will the work be finished?

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SSC CGL 2022 Tier-I Official Paper (Held On : 13 Dec 2022 Shift 3)
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  1. 9 h
  2. 7 h
  3. 6 h
  4. 8 h

Answer (Detailed Solution Below)

Option 2 : 7 h
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PYST 1: SSC CGL - General Awareness (Held On : 20 April 2022 Shift 2)
25 Qs. 50 Marks 10 Mins

Detailed Solution

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Given:

Person A can sanitize a building = 6 h

Person B can sanitize a building = 10 h

Concept used:

Efficiency × Time = Total work

Calculation:

Efficiency Person Time Total work
5 A 6 30
3 B 10

Now, according to the question:

⇒ (5 + 3) = 8 units = 2 hours

⇒  2 hours = 8 units 

so

⇒ 6 hours = 8 × 3 = 24 units

till 7th hours, work completed = 24 + 5 = 29 units

Remaining work = 30 - 29 = 1 unit 

1 unit of work is completed by B in 1/3 hour.

Total time is taken to complete the total work = 7 hours

∴ The correct answer is 7 h.

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