A 4 digit code - Math Central

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Question from chris, a student:

I saw a previous question about a 4 digit code type thing and I was wondering if you could email me that same type of thing.
I changed the pin number on my phone's lock screen and I tried hard to remember it, then I had to go help my mom with something and I automatically forgot it and there is no way for me to get back on my phone.
I need all of the possible 4 digit number combinations without the numbers 0, 1, or 2. It cannot be repeating, ascending, or descending.
I know you may not want to do all of them, but I really need it.

You're right, I don't want to. You're going to!

Write down 333, and try

3333
3334
...
3339

Then change the tens digit and try

3343
3344
...
3349

When you get to 3399, change the hundreds digit and start again at 3433, and so on. You have 2401 numbers to try - allow yourself about eight to ten fifteen-minute sessions and don't work for too long at a time.

Good hunting!
RD

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