Let, speed of car B is -x.
So, speed of car A is x + 10.
Relative speed of A w.r.t B is ,
[tex]v_{ab}=(x+10)-(-x)\\\\v_{ab}=2x+10\ mph[/tex]
So, time taken by A to travel 300 miles relative to B given by 3 hours .
[tex]v_{ab}=2x+10\\\\\dfrac{300}{3}=2x+10\\\\x = 45 \ mph[/tex]
Therefore, speed of car A is 45 + 10 = 55 mph.
Hence, this is the required solution.