Explanation:

- Let's start by understanding the problem: 60 men can complete a work in 'D' days.

- When 40 men do the same work, it takes them 12 extra days, so they finish in D + 12 days.

- Calculate the total work in man-days: 60 men * D days = 60D.

- Now, for 40 men, work done is: 40 men * (D+12) days = 60D.

- From 40(D + 12) = 60D, solve to find D = 24 days.

- Work done by 12 men in days for 3/5th work: Total work = 60D, so 3/5th = (3/5) * 60D.

- Convert to man-days: 36D * 3/5 = 36 * 24 * 3/5, to be done by 12 men.

- Days required = (36 * 24 * 3/5)/12.

- Simplify to find: 72 days.

Correct Answer: Option 1 - 72 days

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