The Math Forum

Ask Dr. Math - Questions and Answers from our Archives
Associated Topics || Dr. Math Home || Search Dr. Math

Let's Make a Deal Probability

Date: 06/01/99 at 11:15:53
From: Brion McGinn
Subject: Let's Make a Deal Probability

In the TV show "Let's Make a Deal" they show you 3 doors, of which you 
pick one. Then Monty shows you what's behind one of the doors you did 
not pick, and the door he shows you is a loser. Then he asks you if 
you would like to change doors. From a probability standpoint, should 
you change doors? In other words, does either the act of his showing 
you a door or his knowing the answer when he shows you a door affect 
the probabilty?

If there is no effect of his knowing or showing, you start out 
choosing with a 33% chance of being right. Then when the door is 
shown, your probability changes from 33% to 50%, and there is no 
benefit to switching or not switching.

However, if his knowing has an influence, then let's look at what 
happens if you choose door number 1.

If door 1 is right (1/3 of time):
    Monty shows you door 2 (1/6 of time)  you should NOT switch
    Monty shows you door 3 (1/6 of time)  you should NOT switch

If door 2 is right (1/3 of time):
    Monty shows you door 3 (1/3 of time)  you should switch

If door 3 is right (1/3 of time):
    Monty shows you door 2 (1/3 of time)  you should switch

Following this, assume Monty shows you door 2... then 1/3 of the total 
times you should switch, and 1/6 of the total times you should not 
switch. Or, you are twice as likely to be right by switching than by 
not switching.

Which of the two circumstances is the case? Is it, or is it not 
beneficial to switch?

Date: 06/01/99 at 11:45:08
From: Doctor Mitteldorf
Subject: Re: Let's Make a Deal Probability

Dear Brion,

You're right that it depends on what Monty knows and what he intends. 

Take a look at our FAQ answer at   

Here's another paradox, from my friend Uri Wilensky, that you might 

You're shown two envelopes with money inside, and you are told that 
one envelope has twice as much money as the other. You can pick A or 
B. You pick A, and open it, to find $100.

Now you're given the opportunity to switch. If you switch, you have a 
50% chance of losing $50 and a 50% chance of gaining $100. So, it 
would seem that switching is the thing to do.

But, wait a minute - you have no new information that you didn't have 
before. Can it really be that the "second envelope" usually gives you 
a better prize than the first?

- Doctor Mitteldorf, The Math Forum   
Associated Topics:
High School Probability
High School Puzzles

Search the Dr. Math Library:

Find items containing (put spaces between keywords):
Click only once for faster results:

[ Choose "whole words" when searching for a word like age.]

all keywords, in any order at least one, that exact phrase
parts of words whole words

Submit your own question to Dr. Math

[Privacy Policy] [Terms of Use]

Math Forum Home || Math Library || Quick Reference || Math Forum Search

Ask Dr. MathTM
© 1994- The Math Forum at NCTM. All rights reserved.