I got an interesting question from a college instructor last week asking why the core excludes the divisibility rules. I wrote back with a comment that the core is very consistent in avoiding tricks or memorized systems without understanding. She wrote back with a very well reasoned response about the rules being based on place value and the properties of operations. She felt that if the rules were taught correctly and not through songs, games, or other “shiny stuff” they would serve students well. So, we are now considering adding the divisibility rules to our curriculum guides (<a href=”http://schools.utah.gov/CURR/mathelem/Core-Curriculum.aspx” title=”Utah Core Standards”>), with some pretty specific instructions about how they should be taught for understanding. So, Bill and the rest of the forum, what about it? Should the divisibility rules be part of a discussion of LCM and GCF and, if so, when and how?