Divisor 199

To determine if any number is divisible by 199, apply this test:

1. If your number ("X") has 4 digits or more, separate the last (smallest) 3 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 4 digits, L = 0 and therefore R = X.
2. Multiply L by 5 and add to R.
3. Take that result and cross off its final digit (units). Take this new number and add 20 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 199. That is, your original number is divisible by 199 if (and only if) Y is. Now that it's much smaller, with basic knowledge of your 199-times tables, it should be easy to visually see if Y is divisible by 199. If the Y is still much larger than 199, the above process can be repeated until it does reduce to within small multiples of 199.

Easy!