A while back John Stevens asked me if I would write a guest post for Table Talk Math. Of course I was happy to do so. I knew I wanted to share the process of exploring a trick to figure out the math behind it so parents would be encouraged to engage their kids in asking why and building on conceptual knowledge rather than being tempted to tell kids a procedure. After some considering I opted against any of the tricks in the book and I decided on the nines trick. You know the one where you use your fingers? This one:
I had a great time figuring out why it works and I don't want to rob you of that opportunity to explore! After you've played feel free to read my guest post and leave a comment if you explained it differently.