Mm, Good Maths

A grade 11 girl, with an accounting exam straight after her lesson.  Working on the 2nd violin part of the 2nd movement of Bach's Brandenburg Concerto #3, which is in compound duple time.  The last bar has 2 tums (dotted crotchets) - she wasn't playing either of them long enough.

Me: How many tis fit into these notes?
Girl: [promptly]: One and a half.
Me: If we were counting tas, sure.  How many tis fit into a tum?
Girl: Two [I shake my head]... Four [I shake my head again]
Me [pointing to the 6 in the time signature, rather pointedly]: How many tis in every bar?
Girl [pausing]: Ummm... 6?
Me: And if this bar has 2 notes of equal rhythm, they have to have the same number of tis, right? [girl nods slowly] - So, what's 6 divided by 2?
Girl [promptly]: Four.
Me: Good luck with that accounting exam.
Girl: Oh.  I mean three.  I told you I'm not good with numbers!