Or she was just very nervous. Your cognitive abilities decline if you're put on the spot. Have you ever said something stupid while speaking in front of a group?

The girl goes to Syracuse, which is a somewhat selective University. She would not have gotten in if she couldn't do the math (her SAT scores simply wouldn't be high enough.)

This question would have been easy for me and actually would have probably boosted my confidence during the interview. But that's because I'm a quantitative person. Throw a question I was completely unexpecting though and I could be stuttering and saying stupid things. I'm sure there would be some questions or exercises the author could complete far more smoothly than I could.

Calling her innumerate (and making the analogy of her being illiterate and thus incompetent) for this slip makes me think you haven't been on many interviews yet.