Divisor 641

To determine if any number is divisible by 641, apply this test:

1. If your number ("X") has 6 digits or more, separate the last (smallest) 5 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 6 digits, L = 0 and therefore R = X.
2. Multiply L by 4 and add to R.
3. Take that result and cross off its final digit (units). Take this new number and subtract 64 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 641. That is, your original number is divisible by 641 if (and only if) Y is. Now that it's much smaller, with basic knowledge of your 641-times tables, it should be easy to visually see if Y is divisible by 641. If the Y is still much larger than 641, the above process can be repeated until it does reduce to within small multiples of 641.

Easy!