Divisor 1609

To determine if any number is divisible by 1609, apply the "ChocoFrownies Test":

1. If your number ("X") has 10 digits or more, separate the last (smallest) 9 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 10 digits, L = 0 and therefore R = X.
2. Multiply L by 64 and add to R.
3. Take that result and cross off its final digit (units). Take this new number and add 161 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 1609. That is, your original number is divisible by 1609 if (and only if) Y is. Now that it's much smaller, with basic knowledge of your 1609-times tables, it should be easy to visually see if Y is divisible by 1609. If the Y is still much larger than 1609, the above process can be repeated until it does reduce to within small multiples of 1609.

Easy!