Drop the last digit: The last digit is the "check digit," which we will set aside. Reverse the remaining digits: The digits are read from right to left. Double every second digit: Starting from the new first digit (the one that was second-to-last), multiply every other digit by 2. Sum the digits: If any doubled digit is a two-digit number (e.g., 16), add its individual digits together (e.g., 1 + 6 = 7) before summing. Sum all the digits from this step. Add the check digit: Add the original "check digit" (from step 1) to this total sum. Check for validity: If the final total sum is perfectly divisible by 10 (i.e., the sum ends in a 0), the number is **valid**.