Marilyn vos Savant writes a weekly column called “Ask Marilyn” for Parade magazine. In her column of August 18, 2013, she proposed the following problem. Five cats are in a sack: two are tabbies and three are calicos. You let one cat out of the bag, but it runs up a tree before you get a chance to see its color. Then you purposely let out another cat and observe it to be a tabby. What is the probability that the cat in the tree is also a tabby? Marilyn’s answer is 1 chance in 4.

Many readers wrote to Marilyn, arguing that the correct answer should be 2 out of 5 and that observing the one tabby does …