Two eighth-grade classes are selling chocolate bars to raise money for an end-of-year field trip. One class is selling chocolate bars for $3.00 each and has already raised $275.00. The other class is selling chocolate bars for $3.50 each and has already raised $250.00. Which equation can be used to calculate c, the number of additional chocolate bars each class needs to sell, so that the total amount raised is the same for both classes?