I don't think expressing the computation in fractions "explains" the problem well enough here. It simply shows that the end results ($25 and $24) are different, which is also stated in the text anyway. The problem is in the (false) intuition that selling "2 per $1 and 3 per $1" must be equivalent to selling "5 per $2" in this particular situation.

(But in general it is not meant to be a complicated puzzle, indeed).