A coffee shop owner wants to create a custom blend using Colombian beans and Ethiopian beans. She uses 10 pounds of Colombian beans and 15 pounds of Ethiopian beans. The total cost of the blend is $300. The equation $10c+15e=300$ represents this situation, where $c$ is the cost per pound of Colombian beans and $e$ is the cost per pound of Ethiopian beans. Which of the following is the best interpretation of $e$ in this context?