Is This Bathroom Occupied?

Friday, August 11th, 2017

Last Friday, FiveThirtyEight's Friday puzzle column, The Riddler, posed a fun, self-contained puzzle. I'll let the author, Oliver Roeder, explain:

There is a bathroom in your office building that has only one toilet. There is a small sign stuck to the outside of the door that you can slide from “Vacant” to “Occupied” so that no one else will try the door handle (theoretically) when you are inside. Unfortunately, people often forget to slide the sign to “Occupied” when entering, and they often forget to slide it to “Vacant” when exiting.

Assume that 1/3 of bathroom users don’t notice the sign upon entering or exiting. Therefore, whatever the sign reads before their visit, it still reads the same thing during and after their visit. Another 1/3 of the users notice the sign upon entering and make sure that it says “Occupied” as they enter. However, they forget to slide it to “Vacant” when they exit. The remaining 1/3 of the users are very conscientious: They make sure the sign reads “Occupied” when they enter, and then they slide it to “Vacant” when they exit. Finally, assume that the bathroom is occupied exactly half of the time, all day, every day.

Two questions about this workplace situation:

  • If you go to the bathroom and see that the sign on the door reads “Occupied,” what is the probability that the bathroom is actually occupied?
  • If the sign reads “Vacant,” what is the probability that the bathroom actually is vacant?

Extra credit: What happens as the percentage of conscientious bathroom users changes?

I submitted my answer (62.5% and 75%, respectively), and ended up visualizing the "extra credit" question (code on Github), which made it into the next week's Riddler. Hooray fifteen minutes of nerd fame!

As well, I typed up a simple explanation of the solution in LaTex, which can also be found on Github in .pdf or .tex format.


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