Math CentralQuandaries & Queries


Question from Patricia, a teacher:

Every prime number greater than 10 has a digit in the ones place that is included in which set of numbers below 1 3 7 9 or 1 3 7 or 0 2 4 5 6 8 or 1 3 5 9

There are two parts to my strategy.

  • I can eliminate some lists, because I know there are factors for them: ending in 2 means it is divisible by 2 (not prime); ending in 5 means it is divisible by 5 (not prime). These come up because we work with a decimal system (all higher digits represent numbers divisible by 10, and therefore 2 and 5).

  • I can look at a list of primes (e.g. and I see 1, 3, 7, 9.

This answers the question, with two sources.


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