White-Tailed Cats

“I see you’ve got a cat.” said Ms Jones to Ms Smith. “I do like its cute white tail! How many cats do you have?”

“Not a lot,” said  Ms Smith. “Ms Brown next door has twenty, which is a lot more than I’ve got.”

“You still haven’t told me how many cats you have?”

“Well…let me put it like this. If you chose two distinct cats of mine at random, the probability that both of them have white tails is exactly half.”

“That doesn’t tell me how many you’ve got!”

“Oh yes it does.”

How many cats does Ms Smith have — and how many have white tails?

More later,

Nalin Pithwa


  1. Anubhav C. Singh
    Posted August 13, 2015 at 1:43 pm | Permalink | Reply

    I guess 4?

  2. Anubhav C. Singh
    Posted August 13, 2015 at 1:44 pm | Permalink | Reply

    I mean 4 cats with 3 white tails

    • Posted August 13, 2015 at 8:16 pm | Permalink | Reply

      Some basics of probability theory are required to solve this

  3. Posted January 17, 2016 at 5:20 am | Permalink | Reply

    The solution is basic probability as explained in Prof Ian Stewart’s book, Cabinet of Mathematical Curiosities. I will post it after some time in a separate blog.

One Trackback

  1. By Quora on January 16, 2016 at 7:57 pm

    Is there a mathematical approach to the solution of https://mathhothouse.wordpress.com/2015/08/12/white-tailed-cats/?

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