.How do I start with this problem?

Question

You and a friend take turns driving 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 hour?

Answer 1

d=r*t

450=60*3.5+r*4

450=210+4r

450-210=4r

240=4r

r=240/4

r=60 mph