Problem of the day for Tuesday, November 13

Last Friday's problem

November 8 hints and answers

How many different four-digit numbers can be formed using the digits 1, 1, 9, and 9?


We're going to be counting things for a few days now. And for many of these problems it will help to make a (systematic) list of possibilities.

For this problem, a systematic list is just what's called for.

As always, be sure to explain the work you did to arrive at your answers!