Divisor 20123

To determine if any number is divisible by 20123, apply the "StormRider 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 615 and subtract this from R.
3. Take that result and cross off its final digit (units). Take this new number and add 6037 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 20123. That is, your original number is divisible by 20123 if (and only if) Y is. Now that it's much smaller, with basic knowledge of your 20123-times tables, it should be easy to visually see if Y is divisible by 20123. If the Y is still much larger than 20123, the above process can be repeated until it does reduce to within small multiples of 20123.

Easy!