18，905→1，865 →161→11
so it is not a multiple of 17, but for 19, the test gives the opposite conclusion:
18，905→1，900=100×19
Armed with this battery of tests, you can readily check the primality of all numbers up to 500(as 232=529 exceeds 500, so19 is the largest potential prime factor that you need concern yourself with). For example, to settle the matter for 247, we just need to check for divisibility up to the prime 13(as the square of the next prime,172=289, exceeds 247). Applying the test for 13,however, we learn from 247(24+28)=52→13, that we have a multiple of 13:(247=19×13).
