A farm has a 15-acre field and a 40-acre pasture. The total number of animals (cows in the field and sheep in the pasture) on the farm is 1,200. The equation $15c+40s=1200$ represents this situation, where $c$ is the average number of cows per acre in the field and $s$ is the average number of sheep per acre in the pasture. Which of the following is the best interpretation of $s$ in this context?