Amina was in Y3 when she composed this puzzle:

My number:

- is bigger than 289
- is smaller than 310
- is a multiple of 10
- is not a multiple of 100

It was Colleen Young, in her blog “Mathematics, Learning and Web 2.0” ( http://colleenyoung.wordpress.com/ ) who made me think of Amina. Unlike this one, Colleen’s blog is obviously created by someone who knows just what they’re doing; she pointed out that there are a number of books of interest to maths teachers which are available as free or very cheap Kindle downloads.

One of these is Mind Hurdles: Mystery Numbers (Cool Math Games For Mathematically Gifted Kids). L L Kross’ Mystery Numbers are just the same as what Amina and I called Number Riddles. I’m afraid that what was a free download when Colleen spotted it now costs you £1.93, but it’s still little enough for a useful resource you can get plenty of mileage from. There are 36 puzzles in all, ranging from simple clues to identify two-digit numbers, up to as many as ten clues for numbers of five and even six digits.

Of course, creating these puzzles and checking that they’re watertight and contain the right amount of information is a lot more demanding than solving them – which is why I was so pleased with Amina’s effort, and why it’s handy to have plenty of examples available.

Incidentally, when I tried a few with two ten year-olds at least a couple of the puzzles seemed insecure. Children rarely object to proving adults wrong, so I don’t think it matters too much, but you may find it helpful to know of the possibility in advance.

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