Understanding the Relationship
This problem involves an inverse relationship between the number of cows and the number of days the grass will last. This means:
* More cows = Less time
* Fewer cows = More time
Calculations
1. Find the total "cow-days": 40 cows * 40 days = 1600 cow-days
This represents the total amount of grass available.
2. Calculate how long the grass will last for 20 cows:
1600 cow-days / 20 cows = 80 days
Answer: The grass will last for 80 days if there are 20 cows.