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 June 26th, 2012, 02:28 AM #1 Member   Joined: Jan 2012 Posts: 52 Thanks: 0 Tricky Probability Problem Hello, This seems to me a tricky probability problem, given that the answer I found is too easy to be correct. Can someone check, please? There are two cards: one of them has both sides in red and the other one has one side in red and the other side in blue. One of the cards is randomly picked and put over a table. If the side of the card showing up is red, give the probability of the other side being red also. My answer is: 1/2. Thanks!
 June 26th, 2012, 05:21 AM #2 Math Team   Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 14,136 Thanks: 1003 Re: Tricky Probability Problem 2/3 same as 4 cards: b,r,r,r ; and you got one of the r's ': probability of picking a r from the remaining 3
June 26th, 2012, 05:24 AM   #3
Math Team

Joined: Dec 2006
From: Lexington, MA

Posts: 3,267
Thanks: 408

Re: Tricky Probability Problem

Hello, tiba!

You are half-right . . . It is a trick question.

Quote:
 There are two cards: one has red on both sides, the other one has one side red and the other side blue. One of the cards is randomly picked and put over a table. If the upper side of the card is red, give the probability of the other side being red also. My answer is: 1/2 [color=beige] . . [/color][color=blue] . . . no[/color]

Let's label the faces of the cards.
[color=beige]. . [/color]Card-1 has $R_1$ and $R_2.$
[color=beige]. . [/color]Card-2 has $R_3$ and $B.$

You choose a card and place it on the table.
You look at the top face of the card; it is red.

There are THREE possible situations:
[color=beige]. . [/color](1) You are looking at $R_1$; the other side is $R_2.$
[color=beige]. . [/color](2) You are looking at $R_2$; the other side is $R_1.$
[color=beige]. . [/color](3) You are looking at $R_3$; the other side is $B.$

In TWO out of THREE cases, the other side is red.

The probability is $\frac{2}{3}.$

 June 29th, 2012, 01:16 AM #4 Senior Member   Joined: Jan 2010 Posts: 205 Thanks: 0 Re: Tricky Probability Problem Never mind, I asked a really dumb question

 Tags probability, problem, tricky

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