Jim Murphy says:
This test about divisibility reminds me of my grade school maths teacher who gave us these rules for divisibility, up to and including, 12. 2. Is the last number even. (pretty obvious) 3. Add the digits. If the total is divisibly by 3, then the original is. 4. Are the last 2 numbers divisible by 4? Then the whole thing is. 5. Last digit a zero or a five? 6. Divisible by 2 and 3? then the whole thing is. 7. No test. Just divide and suffer. 8. Are the last three digits divisible by 3? 9. Add the digits and divide by 9. Same kinda thing as for 3. 10. Last number a zero? 11. Add alternate digits and compare the sums. If the difference is 0 or 11, then the number is divisible. 12. Is it divisible by 3 and 4? Then it's divisibly by 12. Of course, this is all base 10, but it makes me think I was damn lucky to have a maths teacher who'd teach this stuff to us in grade 6, along with the squares up to 25^2, the cubes up to 12^3, the powers of 2 up to 2^10, powers of 3, prime numbers up to 100, etc, etc. Do kids get this these days? I know I've breathed a prayer of thanks for Bill Alford many times over the years.
See also:
file: /Techref/method/math/divisable.htm, 1KB, , updated: 2005/11/12 00:33, local time: 2019/2/23 20:24,

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