Answer: 4 days
Explanation:
We can divide to find how much of a field each man can clear in one day. Since it takes 12 men 5 days to clear a field, each man can clear 1/60th of a field each day.
If we multiply 15 men by 1/60th of a field per day, we see that the 15 men can clear 1/4th of a field each day, so it would take 4 days for them to clear the entire field.