While we wait for ILMM's math answer, I have one for you. It's called "something that you know that you did not know you knew".
Starting with FDR, try to give the name of every 1st Lady. You will be surprised at how well you do.
So you don't skip over any pres.,here's the list. (If I leave one out I'm sure someone will catch it and post it)
FDR, Truman, Ike, JFK, LBJ, Nixon, Ford, Carter, Reagan, Bush, Clinton, Bush and Obama.
So that's 13. I predict that most of you will remember them all. (I got 'em all exept Bush Jr., due to Apathy)