Divisor 21247

To determine if any number is divisible by 21247, apply the "Alfie Test":

1. If your number ("X") has 7 digits or more, separate the last (smallest) 6 digits from the rest. This makes two smaller numbers, call them L^eft and R^ight (note: don't add in trailing zeros to L). If there are fewer than 7 digits, L = 0 and therefore R = X.
2. Multiply L by 1391 and add to R.
3. Take that result and cross off its final digit (units). Take this new number and subtract 6374 times the digit you just crossed off. Call this final result "Y".
4. Y will be much smaller than X, but we have preserved divisibility by 21247. That is, your original number is divisible by 21247 if (and only if) Y is. Now that it's much smaller, with basic knowledge of your 21247-times tables, it should be easy to visually see if Y is divisible by 21247. If the Y is still much larger than 21247, the above process can be repeated until it does reduce to within small multiples of 21247.

Easy!