##### ###### ##### ### # # ### # # ###### ## ## ## ## ## ## ## # # # # # ## ##### #### ##### # # # # # # # #### ## # ## ## ## ## # # # # # ## ## # ###### ## ### # ### # ######
##### ###### ##### ### # # ### # # ###### ## ## ## ## ## ## ## # # # # # ## ##### #### ##### # # # # # # # #### ## # ## ## ## ## # # # # # ## ## # ###### ## ### # ### # ######
(Building a username)
A username has 2 parts: a first character (must be a letter, 26 options) AND a second character (any digit, 10 options).
How many usernames? You make both choices: pick a letter AND pick a digit. That's the product rule: 26 × 10 = 260 usernames.
Each letter pairs with each digit. The choices are independent. That's when you multiply.
