Age 16 to 18 Challenge Level:
Why do this problem?
A non standard problem on prime numbers and AP's that requires some
careful mathematical reasoning
Let the class work at first individually and then is small groups.
Discuss their findings. If they have not met modulus arithmetic, or
the usual notation, then introduce this notation as a standardised
way of recording the reasoning in this solution.
How can we be sure we have considered all possible cases?