How do I find the equation for this question Jet B takes off

How do I find the equation for this question:

Jet B takes off 1/2 hour after Jet A takes off. If Jet B travels 100 mph faster than Jet A and overtakes Jet A exactly 4 hours later, find the speed of Jet A.

Solution

Let the speed of Jet A be x mph. Then the speed of Jet B is (x + 100) mph.

The distance travelled by Jet A in four and a half hours is the same as the distance travelled by Jet B in 4 hours. Since distance = time * speed, we have (9/2)x = 4 (x +100) or, 9x = 8 ( x + 100) or 9x = 8x + 800 or, 9x - 8x = 800 or, x = 800 . Thus, the speed of Jet A is 800mph.