Final answer:
The student's question relates to the cost of an additional unit when the price per unit is constant. It shows that for each additional unit, the total cost increases by the price per unit, which in this case is $1.
Step-by-step explanation:
The question is asking about the cost of adding one additional unit of a product when the price per unit is constant. Given that the total cost for 20 units is $20, we divide the total cost by the number of units to determine the price per unit, which is $1 per unit ($20/20 units).
Therefore, when you buy 21 units at $1 per unit, the total cost is $21. Thus, from 20 to 21 units, the total cost increases by $1, which is the price of the additional unit. This question is a basic example of calculating costs within the concept of linear cost functions where the variable cost per unit stays constant.