A and B can complete a task in 12 days. However, A had to leave a few days before the task was completed and hence it took 16 days in all to complete the task. If A alone could complete the work in 21 days, how many days before the work getting over did A leave?

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RRB ALP Previous Paper 4 (Held On: 10 Aug 2018 Shift 2)
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  1. 7
  2. 5
  3. 9
  4. 3

Answer (Detailed Solution Below)

Option 1 : 7
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Given A alone can complete the work in 21 days

A and B together can complete the same work in 12 days

⇒ Total work = L.C.M of (12, 21) = 84

⇒ One day work of A = 4

⇒ One day work of (A + B) = 7

⇒ One day work of B = 3

Let A worked for x days and B worked for 16 days

⇒ 4x + 3 × 16 = 84

⇒ x = 9 days

∴ A left the work before (16 - 9 =) 7 days.
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