In Canada, the Canada Revenue Agency considers gambling winnings to be windfalls that are not subject tax, so any winners receive the full value of the advertised prizes [0].
[0] https://www.canada.ca/en/revenue-agency/services/tax/technic...
[0] https://www.canada.ca/en/revenue-agency/services/tax/technic...
https://en.wikipedia.org/wiki/Skill_testing_question
What's funny is that the law doesn't say if you're allowed help, or if you need to answer correctly on your first try, simply that they wrote the correct answer on a receipt which the merchant is supposed to keep. When I worked at McDonalds and it was busy, we were encouraged to just tell people the answer so they could redeem their free muffin and keep the line moving.