Explanation:

- 30 men can harvest the meadow in 7 days.

- This forms the basis that 30 men can complete one unit of work in 7 days.

- Therefore, the total amount of work needed is 30 men-days multiplied by 7 days, which equals 210 man-days.

- We need to find out how many days 21 men will take to complete the same 210 man-days of work.

- If 21 men work, you calculate the number of days needed by dividing the total workload by the number of men: 210 man-days / 21 men = 10 days.

Option 1: 10 days

- Option 2: 14 days - Not correct, because it exceeds 10.

- Option 3: 12 days - Not correct, because 21 men can finish in less than 12 days.

- Option 4: 15 days - Not correct, as fewer days are needed.