Challenge: APK duty and the real inspection cycle

Your neighbour in Putten swears by one number sequence for every car: '4-2-1-1, always, full stop.' The CBR exam designer knows you hear this everywhere — and counts on you not knowing that this rhythm differs per fuel type and eventually switches to annual. The book itself, in fact, names no interval figure at all; only the RDW knows the real, two-branch rhythm.