Explanation:

- Initially, the total amount of work is calculated as 60 men working for 40 days. So, the work is 60 x 40 = 2400 man-days.

- The first 10 days involve 60 men, so work completed = 60 men x 10 days = 600 man-days.

- 5 men leave after every 10 days, reducing the workforce gradually.

- 10-20 days: 55 men work, so 55 x 10 = 550 man-days completed.

- 20-30 days: 50 men work, so 50 x 10 = 500 man-days completed.

- 30-40 days: 45 men work, so 45 x 10 = 450 man-days completed.

- Remaining work = 2400 - (600 + 550 + 500 + 450) = 300 man-days.

- From day 40 onwards, 40 men work. To complete 300 man-days, it takes 300/40 = 7.5 days.

- Total days = 40 days + 7.5 days = 47.5 days.

The correct option is:

Option 1 — 47.5 days