These are the prime numbers between 20 and 100:<p>23,29,31,37,41,43,47,53,59,61,67,71 73,79,83,89,97<p>Evens are out; so are small multiples. A multiple of three might end in 3, 9 or 7. A multiple of 7 may end in 1. So a &quot;conservative mental arithmetic sieve&quot; that wants to make sure one&#x27;s picking a prime number will coalesce around 29 or 37.<p>Myself, I picked 11 and 17.