I don't know but...

I like the fact the OP explicitly stated it was a homework question.

Too many times I've suspected a question of being homework based and it annoys me how it's posted a "real life" problem when it really isn't.

I'm perhaps biased because my worst experience was in an on-line chatroom where some girl was asking me questions about MS-DOS which I unwittingly answered only to be told 1/2 hour later "Thanks I was writing an exam and you helped me with the answers". I felt so abused....