If a car travels at a speed of 25 mi/h for t hours then travels 45 mi/h for m hours what does the expression 25t + 45m represent

Answer :  25t + 45m represent the length that the car travels

In this case, the car is traveling at 2 different velocities, 25miles/hour then 45 miles/hour. The 25 and 45 represent those two velocities.
The car is also driving in different duration, t hours at 25mi/h and m hours for 45mi/h. The t hours and m hours is represent the duration.
If you multiply duration and velocity, you will get the distance the car go.