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Teaser Problem: "The Three Cards"

Fun with statistics

Suppose you have three cards:

• white on both sides
• black on both sides
• black and white sides

You put these into a hat. You draw out one card and place it (without looking at the reverse side) on the table. It is black on top. What is the probability that the other side is also black?

I'll post the answer on or about February 1. Or, you can email me at for the answer.

This teaser problem from "The Three Cards" in Entertaining Science Experiments with Everyday Objects by Martin Gardner, (republished by) Dover Publications, Copyright 1960.

—John Schuyler, January 1998.

Copyright © 1998 by John R. Schuyler. All rights reserved. Permission to copy with reproduction of this notice.