Cake
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    • My kids are learning from Beast Academy (in homeschool) and it's full of neat and useful ways to play with numbers.

      Perhaps my favorite so far is a way to quickly determine if any number is divisible by 9, 6, and/or 3.

      Say, this number:

      1112172

      (The photo below has my daughter looking at Latin, not math, but my husband's reaction is pretty much how I feel when I discover my kids have learned a shortcut I don't know. ;) )

    • I felt old as a teacher when high school students told me that they didn’t know the 3,6,9 rule. I didn’t use calculators in school until high school and remember learning that shortcut in middle school: I suppose it was more useful back then when you didn’t have a smartphone handy.

      You know, I wasn’t sure of the age range of your homeschooler, but they might enjoy this game that @Mathgarden has used with his daughter. He says that knowing prime numbers is foundational, but I just think it’s a fun and addictive little game. Don’t try to beat his high score: last I heard, he got 56 correct in one minute(!).