There are a couple of ways to think about this.
1) Weight the speed by the time and compute the weighted average.
(60*2.5 + 75*1.5)/(2.5 + 1.5) = 65.625
Her average rate of speed is 65.625 mph.
2) Consider distance and time.
distance = speed * time
distance₁ = speed₁ * time₁
distance₂ = speed₂ * time₂
total distance = distance₁ + distance₂
total time = time₁ + time₂
And the average speed is given by
average speed = (total distance)/(total time)
Using the above formulas, we have
distance₁ = 60 mi/h * 2.5 h = 150 mi
distance₂ = 75 mi/h * 1.5 h = 112.5 mi
total distance = 150 mi + 112.5 mi = 262.5 mi
total time = 2.5 h + 1.5 h = 4.0 h
average speed = (262.5 mi) / (4.0 h) = 65.625 mi/h
Bette's average rate of speed is 65.625 mph.