P and Q can complete a project in 15 days and 10 days, respectively. They started doing the work together, but after 2 days, Q had to leave and P alone completed the remaining work. In how many days was the whole work completed?

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SSC CGL 2022 Tier-I Official Paper (Held On : 01 Dec 2022 Shift 1)
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  1. 11
  2. 12
  3. 13
  4. 10

Answer (Detailed Solution Below)

Option 2 : 12
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Detailed Solution

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(P + Q)'s 1 day's work = 1/15 + 1/10

⇒ (2 + 3)/30

⇒ 5/30

⇒ 1/6 

Work done by P and Q in 2 days = 1/6 × 2

⇒ 1/3

Remaining work = 1 - 1/3

⇒ 2/3

Now,

1/15 work is done by P in 1 day.

So, 2/3 work will be done by P in 15 × 2/3 = 10 days

So, the total time is taken = 10 + 2

⇒ 12 days

∴ The whole work was completed in 12 days.

Alternate MethodCalculation: 

LCM of 15, 10 is 30

Efficiency of P = 30/15 = 2

Efficiency of Q = 30/10 = 3

Two days of work will be (2 + 3) × 2 = 10

Remaining work = 30 - 10 = 20

As Q left remaining work will be done by P = 20/2 = 10 days

Total work done in 10 + 2 = 12 days

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