posted by Leigh on .
A semi-truck travels on a paved road for 2 hours at an average speed of 20 miles per hour faster than it travels on a dirt road. The time spent on the dirt road is 3 hours. If the entire trip is 200 miles, how fast is the truck traveling on the dirt road?
Use d = r * t for both the paved and the dirt roads. Use x for the unknown speed on the dirt road.
For the paved road, use d1 (distance 1):
d1 = r * t
d1 = (x + 20) * 3
d1 = 3(x + 20)
For the dirt road, use d2 (distance 2):
d2 = r * t
d2 = x * 3
d2 = 3x
Also, you know that the distance traveled on the paved road, d1, and the distance traveled on the dirt road, d2, sum to 200 miles. Thus, you know:
d1 + d2 = 200
See if you can take it from there.