What you need is a definition of average speed. Here it is:
Average speed = (distance covered) / (time to cover the distance) .
For the race car in the question:
Average speed = (200 mile) / (2.5 hour)
= (200 / 2.5) (mile/hour)
= 80 miles per hour .
That doesn't seem all that fast for a race-car.
But the math is bullet-proof.
