In Common Core math, it often is not good enough to get the correct answer. Instead, students are required to show “higher order” thinking skills — in this case, use of the associative property. Yes, the associative property is important and should be taught at some point. Unfortunately, we suspect that many 7-year olds will not be able to understand this particular assignment. With limited days in the school year, wouldn’t second graders — second graders! — be better off spending their time attempting to master the traditional subtraction algorithm?
In case you were wondering, the use of the term “friendly numbers” by the teacher above bears no relation to the use of the same term in number theory.