See the answer and an explanation here.

The Monty Hall Problem has stumped countless people throughout history, including scientists and mathematicians. It tends to fool readers because the problem seems genuinely random — but the fact that the game show host knows which door contains the prize is key. After the problem appeared in “Parade” magazine in 1990, approximately 10,000 people wrote in to dispute the solution. Of those people, around 1,000 had PhDs.

Stay Curious!

Originally published on Daily Curiosity