Maybe a 12-year old is ready for that.  That seems a little heavy for an 8-year old - calculating out cost of production.  I just sat next to my elementary teachers at a community breakfast yesterday and they reminded me we did the number line, addition and subtraction in 3rd grade (8).  We did multiplication tables in 4th grade (9).

Gotta start somewhere.  I started to catch on to cost of production when i was about 11 or 12 sitting with dad in the combine, doing math with his pocket calculator writing it down on his daytimer.