A certain car increased its average speed by 5 miles per hour in each successive 5-minute interval after the frist interval. If in the first 5-minute interval its average speed was 20 miles per hour, how many miles did the car travel in the third 5-minute interval?

My answer is 30 miles per hour. Based on common sense. Is there a formula someone can show me to set up similar type problems so I will be able to do these problems in an exam. Cheers