The Mod 10 formula doubles the values of alternate digits of the primary account number starting with which digit?

The Mod 10 formula, also known as the Luhn algorithm, is a checksum formula used to validate various identification numbers, such as credit card numbers. According to the method, you start with the digit that is in the second position from the right. This means that you begin by doubling the value of every second digit, starting with the digit immediately to the left of the check digit (the last digit of the primary account number).

Following this rule, the sequence of doubling goes as follows: if you take a primary account number and consider it starting from the right, the first number is not doubled, the second number is doubled, the third is not, the fourth is doubled, and this pattern continues until you reach the leftmost digit.

This method is critical for checking the validity of the number, as it helps detect errors that may have been introduced during data entry. Thus, the correct answer is indeed based on starting from the second digit from the right, ensuring proper application of the Mod 10 validation process.