Answer: The will be 1200 miles apart after 2.4 hours.
First draw a picture of an angle of 60 degrees. The vertex is the starting point of the plane and they are flying away from it.
If you drew points for the new positions of the plane and connected them, you would have triangle. The planes are flying at the same speed, so they would be isosceles triangles. And since the vertex angle is 60 degrees, the other angles have to be 60 as well. This makes it an equilateral triangles, meaning all the sides are the same.
So the distance between the planes is equal to the distance that they have flow.