3. (10 points) There is a pasture, 10 cows can finish the grass in 8 days; 15 cows, if one less cow starts from the second day, can finish in 5 days. Then the grass that grows on the pasture every day is enough for $\qquad$ cows to eat for one day.


【Analysis】By changing the perspective, 15 cows, if one less cow starts eating from the second day, can finish eating in 5 days, can be converted into 13 cows eating for 5 days to solve the problem.
【Solution】Solution: According to the problem, we have:
$$
10 \times 8-(15+14+13+12+11)=15 \text { (units). }
$$
15 cows, if one less cow starts eating from the second day, can finish eating in 5 days, can be converted into 13 cows eating for 5 days.
$$
15 \div(8-5)=5 \text { (units) }
$$

Therefore, the answer is: 5