Question

A crash test dummy is inside a test car moving at 55 km/h. How fast is the dummy moving relative to the seat he is sitting in?

Answer 1

The situation is exactly the same as YOUR speed relative to the seat you're sitting in, aboard the passenger jet flying at 500 mph with you in it.

Answer 2

The dummy is moving with a speed 0 km/h relative to the seat in which it is sitting.

If the relative speed was non-zero, the dummy would move away from its seat, which contradicts the problem formulation.