A store sells apples for $2.50 per pound and oranges for $1.75 per pound. On a particular day, the total revenue from apples and oranges was $20.00. The equation $2.5a+1.75o=20$ represents this situation, where $a$ is the number of pounds of apples sold and $o$ is the number of pounds of oranges sold. Which of the following is the best interpretation of $o$ in this context?