The first car is going 60 miles per hour because if the second car is going at 45 mph, after 4 hours it has already travelled 180 miles. In order for both cars to meet from a total distance of 420 miles, the first car has to travel 420 miles - 180 miles meaning that the first car travelled 240 miles. When you look back at how it took 4 hours for both cars to meet each other, this also means that the first car traveled 4 hours. So the car traveled 240 miles in 4 hours meaning that it went 240miles/4hours or 60mph.