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Double Time

Crack this code which depends on taking pairs of letters and using two simultaneous relations and modulus arithmetic to encode the message.

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Modular Fractions

We only need 7 numbers for modulus (or clock) arithmetic mod 7 including working with fractions. Explore how to divide numbers and write fractions in modulus arithemtic.

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What is the smallest perfect square that ends with the four digits 9009?


Age 16 to 18 Challenge Level:

This example is merely an introduction to the subject to introduce the idea of devising formulae with corresponding deciphering formulae to encipher and decipher text automatically.