Hi Morgan,

There are 10 digits, 0 to 9, and hence for a 4 digit code you have 10 choices for the first digit, 10 choices for the second digit, 10 choices for the third digit and 10 choices for the fourth digit. Thus you have $10 \times 10 \times 10 \times 10 = 10,000$ possible 4 digit codes. Too many for me to list but I can show you how to list them.

You can list them in numerical order the first is 0000, the second is 0001, followed by 0002 and on until 0009. The next one is 0010 then 0011 and so on. The largest is 9999.

If you remember the old style car odometer, before digital odometers, listing the codes from 0000 to 9999 is like watching the odometer clicking off the miles.

Harley