Silly family joke

Homeschool Mom to son: Ok Son. Train A leaves the station travelling 80 km/hr. Three hours later Train B leaves on a parallel track travelling 120 km/hr in the same direction. How far down the track before Train B overtakes Train A? Solve the question.

Son: Ugh! I need Sherlock Holmes.

Mom: Sherlock Holmes is right here, inside your head. You are Sherlock Holmes.

Son: I think I have amnesia......