The puzzle looks easy. You have 1000, add 40, add 1000, add 30, add 1000 again, add 20, add 1000 again, and finally add 10. No calculator. No paper. Just your mind. That last instruction is the hook. It makes people speed up.
They treat the line like a scoreboard instead of a sequence, and a huge number of them land on 5000. That answer feels neat. It is also wrong.