## Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

$16$ two-letter codes
The first letter can be A,B,C or D. So there are 4 choices for the first. Similarly, the second letter can also be A,B,C or D as repetition is allowed. According to multiplication principle, Number of 2-letter codes = $4\times4 = 16$