So, I've decided to make a to-do list (I can hear the cynical sniggering already
Today I'm sitting in the bat-cave programming, so taking a moment off to write this. The AG just sent me this site - you should introduce you 7+-year-old to some of these tricks
Is it Prime? For starters, not if it ends in an even number, 5, or 0 (that means divisible by 2 or 5). To check further, add all the digits, and keep adding until you have just 1 left. If that is 3, 6, or 9 then it's divisible by 3 and definitely not prime. The site gives an easier way of subtracting from 1000 (or similar). I find easier to take 1 from 1000 leaving 999. Then I don't have to carry at all. Afterwards I can just add the one back to the result !