The principle of this trick is that if 100 ≡ h (mod d) then 100a + b ≡ ha + b
(mod d). For example, to test divisibility by 97, we note that 100 ≡ 3
(mod 97). To apply the principle and see if 27019 is divisible by 97 we can
trim off the leftmost digit (2), multiply it by 3 (6) and add that to the
remaining digits (7019), but shifted in to the right by two places: