Answer: 
 
50mph 
 
Step-by-step explanation: 
 
Say for example car A increases by 10 miles per hour and car B decreases by 10 miles per hour. If car A2 = car B3 then by eliminating a few numbers and finally get to 50 you get the following equation. 
 
A = 50+10 
 
b = 50-10 
 
A = 60 
 
B = 40 
 
A2 = 120 
 
B3 = 120 
 
120 = 120