A red car and a blue car are traveling at the same speed. The red car drives 4 hours. The blue car drives an additional 45 minutes and goes 21 more miles than the red car. How fast are the cars going?
Let the speed of the two cars be S (mph)
So......the distance the red car goes in 4 hours + 21 miles = the distance the blue car goes in 4 + 3/4 hours = 4.75 hours
4S + 21 = 4.75 S subtract 4S from each side
21 = .75S divide both sides by .75
S = 28 (mph)
Another way to see this is that the blue car covers 21 miles in 3/4 hr
So 21/ (3/4) = 84 / 3 = 28 mph