Acar travels 200 miles in the same time that a train travels 300 miles. the speed of the train is 20 miles per hour more than the speed of the car. which equation could be used to determine the speed of the car, r, in miles per hour?

The equation is 
time= distance/rate
and the train is going 300 miles 
the car is going 200 miles
the train goes 20 miles faster than the car
fitting the numbers into the equation 
200/r=300/(r+20)
cross multiply 
200/r = 300/(r+20)
200(r+20) = 300r 
distributive property 
4000+200r=300r
subtract 200r from both sides to add like terms 
4000=100r 
divide 100 from each side to get r by itself
40 = rate of car
60 = rate of train