Hopefully you know how these tricks work already. It takes non-mathies a LONG time to discover how the following trick works!! (and usually they have to ask one of us to figure it out):
My students never figured out how the crystal ball read their minds. They thought there was a microphone on my laptop cart, so I started speaking in code and misdirection. Then they claimed it was photo-sensitive (although they didn’t use that term) because I was making shadows on the screen (not a Smart Board, mind you). Fun times.
You have 10a + b – a – b = 9a, so you need to have the same result for all multiples of nine (9, 18, 27, …)
My students never figured out how the crystal ball read their minds. They thought there was a microphone on my laptop cart, so I started speaking in code and misdirection. Then they claimed it was photo-sensitive (although they didn’t use that term) because I was making shadows on the screen (not a Smart Board, mind you). Fun times.
It’s simple all the numbers you will get will have the symbol like the one on 9. Because they gave that symbol to every possible outcome.