Answer:
30 miles per hour
Explanation:
We want to know the average speed over a course with one leg run at one speed, and another leg run at a different speed.
Setup
To find the average speed, we need to know the total time and the total distance. The relation ...
time = distance/speed ⇔ distance = speed × time
can be used to fill in the blanks.
The first leg is a distance of 140 miles. The time for that leg is ...
t1 = (140 mi)/(28 mi/h) = 5 h
The second leg takes 2 hours at a speed of 35 mph. The distance covered is ...
d2 = (35 mi/h)(2 h) = 70 mi
The total distance is ...
d = d1 +d2 = 140 mi +70 mi = 210 mi
The total time is ...
t = t1 +t2 = 5 h + 2 h = 7 h
Solution
The average speed is the ratio of total distance to total time:
d/t = (210 mi)/(7 h) = 30 mi/h
Susan's average speed for the trip was 30 mph.