Problem 63.

Emily takes a trip, driving with a constant velocity of 90 km/h to the north except for a 30 min rest stop. If Emily's average velocity is 75 km/h to the north, how long does the trip take?

Solution:

The average velocity is the ratio of traveled distance and the traveled time:

If the traveled time is t, then Emily traveled time (t-0.5)h with the speed 90 km/h. Then the traveled distance is

We substitute this relation in the first equation and obtain:

Then

Then we can find L (length of the trip):