On a highway trip, Joe drives the first 25 miles at 55 mph, and the next 15 miles at 70 mph. What is his average speed for the trip?

Answer 1

Distance = rate * time

For the first 25 miles he drove at 55 mph, so the time he drove is

d = r * t

25 = 55t₁

.4545 hours = t₁

For the second 15 miles he drove 70 mph

d = r * t₂

15 = 70t₂

.214 hours = t₂

The total distance he drove is 25 + 15 = 40

The total time he drove is t₁ + t₂ = .4545 + .214 = .6685 hours

So the average speed for the trip was

d = r * t

40 = .6685 r

59.84 mph = r