I love showing this to students as a treat after a tough lesson; their responses can be hilarious, if laced with expletives. It’s great to find a fun maths trick that has child-friendly graphics (uncommon) but that can also be extended to enable students to build on, and extend, what they know (rare).

*Words of warning*: many students struggle with the initial calculation of ‘pick a number, find the sum of the digits, subtract from the original number,’ which can spoil the surprise for them. I often do the calculation with them on the board, but they tend to be suspicious that I am communicating with the monster (awww).

I suspect it reflects more on me than on my students that I was much more excited by the fact that all numbers derived by ‘pick a 2-digit number, sum the digits, subtract from original’ are multiples of 9 than I was by the “s**t scary” monster.

More than anything, I’ve been fascinated by which pupils have been able to identify how the trick works – there has been no correlation to ability, age or confidence. I love getting these reminders that I shouldn’t presume what my students can do, even those who I feel I know quite well. I’d love to know how your students respond to it.

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