There's plenty about the notion of buying and selling that can't be understood by any student. An appalling number of kids in public school aren't able to answe

There's plenty about the notion of buying and selling that can't be understood by any student. An appalling number of kids in public school aren't able to answer questions like "how do you make change for $5.67 if you're given a $10 bill." Differentiated instruction is a crock; a teacher who fails to be able to teach a classroom of roughly equal abilities is going to fail even harder to teach a classroom of mixed abilities. Giving extra worksheets with slightly harder problems doesn't cut it, when a significant minority of kids are one or more grade levels above grade (and a significant majority of kids are one or more level below grade).