Some of the firm’s cost, called variable costs, change as the firm alters the quantity of output produced. Thelma’s variable costs include the cost of lemons,sugar,paper cups, and straws: The more lemonade Thelma makes, the more of these items she need to buy. Similarly, if Thelma has to hire worker to make more lemonade, the salaries of these worker are variable costs. The fourth column of the table shows Thelma’s variable cost. The variable cost is 0 if she produces nothing,$0.30 if she produces 1 glass of lemonade,$0.80 if she produces 2 glasses, and so on.