You know what else teaches basic arithmetics? elementary school. And numbers are taught alongside the arithmetics to calculate with them.

So you first learn addition and substraction of single digit numbers, then it goes to double digit numbers, then you learn the multiplication tables and through that divisions. Then you move on to larger numbers, written additions, subtractions, multiplication and division with remainder.

So by the time a child learns the number 80 it also learns how to do arithmetics in that area.