Lizzie came up with a divisibility test for a certain number m /neq 1:
Break a positive integer into two-digit chunks, starting from the ones place. (For example, the number 354764 would break into the two-digit chunks 64-47-35)
Find the alternating sum of these two-digit numbers, by adding the first number, subtracting the second, adding the third, and so on. (In our example, this alternating sum would be 52).
Find m and show that this is indeed a divisibility test for m (by showing that n is divisible by m if and only if the result of this process is divisible by m).