A company finds that there is a linear relationship between the amount of money that it spends on advertising and the number of units it sells. If it spends no money on advertising it sells 100 units. For each additional $2500 spent, an additional 25 units are sold. If x is the amount of money that the company spends on advertising, find a formula for y, the number of units sold as a function of x. (do not use commas in your formula)