Two cars are 400 miles apart when they start moving towards each other. The speed of the first car is 30 mph. The the second car moves 20 mph faster than the first car. After how many hours will the cars meet?

Mrs. morton has a special reward system for her class. when all her students behave well, she rewards them by putting 3 marbles into a marble jar. when the jar has 100 or more marbles, the students have a party. right now, the the jar has 24 marbles. how could mrs. morton reward the class in order for the students to have a party?