Problem: A company conducted a series of promotions for its employees, in order to increase sales. After three promotions, the company's sales increased to $2,000 per day. After five promotions, the company's sales increased to $2,800 per day. After seven promotions, the company's sales increased to $3,800 per day. What is the average rate of change in amount of sales (in dollars) per promotion from the third to the fifth promotion?