Frame of reference
There are three obvious inertial reference frames that can be used to determine when the cars will be even with each other: one where the back car is not moving but the front car is moving backwards at 5 m/s, one where the front car is stationary and the back car is moving forward at 5 m/s, or one where the ground is not moving and the cars are moving at their given velocities. These three perspectives all describe the situation accurately and are physically identical.