Removed self as admin but did not set new password

Hello, I have a Macbook Pro (an older version) but all of the up to date software as of Sept 8. I'm giving my younger brother this computer and getting a new one, so I set up a new account on the computer for him. However, I did not set a password because I was going to let him do that. It prompted me to say this was ok and I did. However, since he is now the admin and I made my own account a standard one, I can not make any changes in preferences because it asks for the admin's password before "unlocking." What would his password be if I did not set one? It is not the one from my account.