Let’s add the two speeds together so we can tell how far apart they are, so they move apart at 1,200 mph. At 3 PM, this is how far away they will be from each other. At 4 PM, they will be double that, which is 2,400. They still need to go 300 miles, which is a quarter of how fast they go in an hour, meaning they need to go for another quarter of an hour. That is 15 minutes, making the time 4:15 when they are 2,700 miles apart, which is answer b.
