Outside a home, there is a 4-key keypad with letters A, B, C, and D that can be used to open the garage if the correct four-letter code is entered. Each key may be used only once. How many codes are...

can you help me understand how to come up with the given solution to this problem?

Extracted text: Outside a home, there is a 4-key keypad with letters A, B, C, and D that can be used to open the garage if the correct four-letter code is entered. Each key may be used only once. How many codes are possible? The number of possible codes is 24