Three coins


You have a set of coins.

  1. One is red on both sides
  2. Another is blue on both sides
  3. The last is red on one side and blue on the other.

If one of the three coins is selected and placed on a table so it can be seen that the top is red, what is the probability the other side is red?









Hint: List the color of each of side of all three coins. Use that to act out the problem and the possibilities.



There are six sides that could be seen: Red, red, blue, blue, red, blue.
1. RR 2. BB 3. RB
If it is known that the side showing is red, that eliminates the BB coin as selected.

That leaves two coins RR or RB as the coin.

However, you don't know which side of the RR coin or if the side of the red side of the RB is showing.

That leaves three possibilities: R B ; R R ; R R
R side showing Blue hidden;
Red side top showing Red side bottom not showing;
Red side top showing Red side bottom not showing

Out of three possibilities how many would be red?

Dr. Robert Sweetland's notes
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