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. Eac

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 _____