Question 443749
Let s = the speed of train A
Then s + 20 = the speed of train B
The distance traveled by train A is:
d = s*t -> d = 4s
The distance traveled by train B will be 500 - d. 
Therefore, 500 - d = (s + 20)t = (s + 20)4
We have two equations in two unknowns.  Since we need to find s, we want to eliminate d:
d = 4s [1]
500 - d = 4s + 80 [2]
Use the value expression for d in [1] and substitute into [2]:
500 - 4s = 4s + 80
8s = 420 -> s = 52.5 
So, the speed of train A = 52.5 mph
The speed of train B = 52.5 + 20 = 72.5 mph