Explanation:

- 25 men can complete the task in 16 days. Thus, the entire task is 25 men x 16 days = 400 man-days.

- In the first 4 days, 25 men work and complete 25 x 4 = 100 man-days of work.

- Remaining work needed is 400 man-days - 100 man-days = 300 man-days.

- 5 more men join, making a total of 30 men working together from the 5th day.

- The remaining work requires 300 man-days / 30 men = 10 days to complete.

Option 1: 10 days

.