Final answer:
Using the concept of cow-days, we determine that a field of grass that feeds 28 cows for 6 days can feed 21 cows for 8 days.
Step-by-step explanation:
If a field of grass feeds 28 cows for 6 days, we can determine how long the same field would feed 21 cows using a simple ratio. We need to find the total number of cow-days that the field can support, which is the product of the number of cows and the number of days. For 28 cows, the field lasts 6 days, so the total cow-days is 28 cows × 6 days = 168 cow-days.
Next, we divide the total cow-days by the number of cows we want to feed, in this case, 21 cows. It gives us 168 cow-days ÷ 21 cows = 8 days. So, the field would feed 21 cows for 8 days.