I think I would set a proportion for this. 1 1/2 hours is 90 minutes. So 90 minutes and 90 miles.
90 300 —- = ——. 90 X
As you can see, miles on top and minutes on the bottom. Now, cross multiply. 90x = 300(90) 90x=27,000 Divide by 90 on both sides to get x alone. 90x/90= x 27,000/90=300