Correct Answer - Option 2 : 36 days

**CALCULATION:**

If Ashit takes 'a' days to complete the whole work, then Biswas will take (a + 12) days to complete the whole work.

As Biswas has completed 60% of the work, the remaining 40% work is completed by Ashit.

∵ Ashit has done 40% of the work before leaving.

∴ Ashit has worked for 2a/5 days.

∵ Biswas has completed 60% of the total work

∴ Biswas has worked for [3(a + 12)]/5 days.

But as Ashit leaves the work 12 days before the Biswas.

⇒ No. of days for which Biswas works = No. of days for which Ashit works + 12

⇒ [3(a + 12)]/5 = (2a/5) + 12

⇒ 3a + 36 = 2a + 60

⇒ a = 24 days

∴ Time taken by Biswas to complete the work independently = (a + 12) = (24 + 12) = **36 days**

- The key to solving this problem is to start from work completed by Biswas.
- The time taken to complete the work ∝ Amount of work.
- Efficiency ∝ 1/Time Taken to complete the work.