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# Code Breaker

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Age 7 to 11

Challenge Level

Many modern codes are based on two very large prime numbers multiplied together.

This problem is based on a code using two different prime numbers less than 10. These two primes have been multiplied together and the resulting number has been used to shift the alphabet forward to new letters, assuming that A is at position 1, B at position 2 etc. For example, if the two prime numbers were 2 and 3, then to make the code, the alphabet would be shifted forward by 6 places. A would become G, B shifts to H and so on.

Which way will you need to shift the letters to decode?

When you have deciphered the code, there will be one word which will remain coded. You can decipher this word by adding the two prime numbers together and shifting the letters again.

Can you find the doubly coded word in this sentence?

JZF SLGP FUDFNHG TE

These two group activities use mathematical reasoning - one is numerical, one geometric.

What happens if you join every second point on this circle? How about every third point? Try with different steps and see if you can predict what will happen.

Work out Tom's number from the answers he gives his friend. He will only answer 'yes' or 'no'.