Today I had a 7-year old student for coding lessons

Really smart - and her English really at 7yo+ level - her mum attended as I had requested.
We worked through Hangman and Bulls and Cows and some other games to program.
Finally I gave her this problem because she was showing me some scales:
20 coins same denomination, and presumably same weight.
But one was fake, counterfeit.
We don't know whether it is heavier or lighter.
How many weighings do you need to identify the fake coin?
And is it heavier or lighter?
How do you do this?
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Mom and daughter conferred and came back with 5+5 which of course halves the coins; the next steps are not so neat, and the final step needs to be negotiated, drag in an eliminated coin. I wonder how such questions challenge the very young. But they were together motivated enough to find a path to solution! Bulls and cows next.
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sydney, New South Wales, Australia

Retired but teaching and studying every day, travelling whenever I can and at home wherever I happen to be. From a small family but wishing I were part of a larger one. My students are scattered all over the world, as is my family. Language is a part [read more]