SORRY... I WAS LOOKING AT THE WRONG SECTION.
I was wondering why it didn't make sense... I'll leave it posted in case someone wants the info.
(this is #33 from the math section)
Each aircraft is traveling at the same rate. At any given time, they will be equal distances away from the starting point.
If you create a triangle with the distance traveled by each aircraft as legs, it will be an isosceles triangle. By definition of an isosceles triangle, the angles at the base of each of those legs are equal.
If you add all the angles together, you get 180 for a triangle. With the angle of divergence between the two aircraft 60, you subtract that from 180 to get the value of BOTH bottom. Because they are equal, you divide by 2 to figure out what they are.
180 - 60 = 120. Divide by 2, and you have 60 for each angle.
Now you know the triangle is not only isoscles, but equilateral (all angles are the same). This also means all sides are the same.
When the aircraft are 1200 mile apart, they are each also 1200 mile from the starting point.
divide distance by velocity to get time.
1200 mile / 500mph = 2-2/5 hours