A thing the question doesn't tell us: What direction is Mr. B walking ?
-- If Passenger B is walking West, in the same direction as the moving walkway, then the stationary observer sees him passing by toward the West at 3.5 km/hr.
-- If Passenger B, for some strange reason, is walking BACKWARDS, heading east on the moving walkway, then the stationary observer sees him passing by toward the EAST, at 0.5 Km/hr.
-- We're wondering why the question ever introduced us to Passenger-A. He never figures into the question in any way. He has no effect on Passenger-B, and we never hear from him again.
