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Alison and Charlie are playing a divisibility game with a set of 0-9 digit cards.
They take it in turns to choose and place a card to the right of the cards that are already there.
And so on!
They keep taking it in turns until one of them gets stuck.
Click here to see an example of a game:
Play the game a few times on your own or with a friend.
Are there any good strategies to help you to win?
After a while, Charlie and Alison decide to work together to make the longest number that they possibly can that satisfies the rules of the game.
They very quickly come up with the five-digit number $12365$. Can they make their number any longer using the remaining digits? When will they get stuck?
What's the longest number you can make that satisfies the rules of the game?
Is it possible to use all ten digits to create a ten-digit number?
Is there more than one solution?
Please send us your explanation of the strategies you use to create long numbers.
This problem featured in an NRICH video in June 2020.
Make a set of numbers that use all the digits from 1 to 9, once and once only. Add them up. The result is divisible by 9. Add each of the digits in the new number. What is their sum? Now try some other possibilities for yourself!
Can you explain the strategy for winning this game with any target?
Is there an efficient way to work out how many factors a large number has?