Not quite.

You buy one mouse, presumably a baby one.

Two months later you now have 13 mice.

One month after that your first mouse gives birth again, so you have 25.

One month after that Your first mouse and it's first 12 give birth, so you've got 181.

One month after that your first mouse plus the 2nd set of mice gives birth, so you've got 325.

One month after that your first set +12 gives birth again, etc.

So, every month you have 1+12^n +12^(n-1)... mice, I believe.

Lemme check that, though.

hmmm, apparently not. I'll have to work on this some more, obviously.

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"Science is the poetry of reality." ~ Richard Dawkins