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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!

### Have You Got It?

Can you explain the strategy for winning this game with any target?

### Counting Factors

Is there an efficient way to work out how many factors a large number has?

# American Billions

##### Age 11 to 14Challenge Level

American Billions printable sheet - instructions
American Billions printable sheet - cards

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.

• After two cards have been placed, the two-digit number must be divisible by $2$.
• After three cards have been placed, the three-digit number must be divisible by $3$.
• After four cards have been placed, the four-digit number must be divisible by $4$.

And so on!

They keep taking it in turns until one of them gets stuck.

Alison places the $5$.
Charlie puts down the $8$ to make $58$, which is a multiple of $2$.
Alison puts down the $2$ to make $582$, which is a multiple of $3$.
Charlie puts down the $0$ to make $5820$, which is a multiple of $4$.
Alison now has to choose from $1, 3, 4, 6, 7,$ or $9$ to make a multiple of $5$.
Convince yourself that Alison is stuck, and that Charlie has won.

Play the game a few times on your own or with a friend.

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.