Is it prime?
March 15, 2008 pm31 11:50 pm
Not fancy, computer-type stuff. I factor numbers. Mostly 4-digit, sometimes 5-digit. Permutations of subway car numbers. And sometimes they don’t factor.
8303? Not even. 8+3+3 = 14, so no 3. Not a multiple of 5.
Subtract 303 – 8 = 295. 7, 11, 13 none go into 295, so they don’t divide 8303.
17? 8303 + 17 = 832. 850 – 832 = 18, so 17 doesn’t go in (what?? if all these numbers were multiples of 17, then their sums and differences would be too. 850 is 50 17s. Since we got 18, one of the numbers was off. That would be 8303)
19? 8303 + 57 = 836
836 – 76 = 760. Hurray! 8303 is not prime.
I find it easy to amuse myself for a full subway ride…
JD2718 
I’m not following your approach for 17 and 19. 8303 + 17 = 832 Huh??
I do this with the 4 digit numbers the clock displays as I’m falling asleep. It’s rare that one takes me longer than a minute, and when it does, I know I’m tired and it’s time to look the other way and fall asleep. Basically, it’s the mathematician’s version of counting sheep…