-   To add up the digits you need to start from the last digit, multiply the
    last digit by 1, second to last by 3 and so on.
    It then checks if the product code is valid (total number is a multiple
    of 10).
    Starting at the right-hand digit will make the formula work for a wide variety of product codes.

-   You can find the number digits in a number by using the
    `scratch:length of ()` block unders “Operators”.
    For example: `scratch:length of (2938339392383) //13`

-   You can access a letter (or a digit) at a specified position in a string
    (or number) by using the `scratch:letter () of []` block under “Operators”.
    For example: `scratch:letter (4) of [18392819202910] //9`

-   To access the last digit of the product code, make a variable called
    “index” and set it to the length of the product code.
    Now the last digit of the product code can be accessed by using the
    `scratch:length of ()` block with the index variable and the product code.
