Math question regarding cost per feet?

The question is "The firm charges 100 for the first 10 feet built. After that, the cost of the labor for each succeeding 10 feet is $25 more than the preceding 10 feet, how much would 90 feet cost" So 10 feet is 100 dollars, 20 feet is 125 dollars, 30 feet is 150 dollars etc. I know the answer, it's $300, and I made up an equation, however, I'm wondering what the CORRECT equation would be to figure this formula out since I can't show my work the way that I did the problem. I have to use an equation.