Interesting, but I add this data point ... On a site I manage I ask people to enter 4 plus nine (in digits). Recently I've had the following answers: thiteen,

Interesting, but I add this data point ... On a site I manage I ask people to enter 4 plus nine (in digits). Recently I've had the following answers: thiteen, 13, tirteen, thirten, thurteen, and 11. Yes. 11. The question is, if someone can't add 4 and 9 do you want their opinion?