Question 892020
You and a friend take turns driving on a 450 mile trip. Your friend drives for 3.5 hours at an average speed of 60 miles per hour. What must your average speed be for the remainder of the trip if you want to reach your hotel in 4 more hours.

My work: 

60 x 3.5(the hours te friend drove) = 210
This much is correct, your friend drove 210 miles.
That leaves 240 miles to go (450 - 210)
-------------
To go 240 miles in 4 hours:
---
d = r*t
r = d/t = 240/4
=======================================

4/210 = 52.5 
4/210 is not 52.5
4/210 =~ 0.019, a meaningless number for this problem.
52.5 miles/hour
---
Include the units in your calculations.
4 hr/210 miles = 0.019 hours/mile ???? No one wants to know that.

However my brother checked it and told me it was wrong and I'm not sure what I did.