How to find Administrator username and password when you set up you MAC
How to find the Administrator username and password when you initial set up your MAC
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The very first account you create is the administrator. The password you gave to that account is the administrator's password.
You can look at System Preferences -> Users & Group and each account that is an administrator will have "Admin" under the username.
If you have forgotten the password for the administrator's account, you cannot get it back. It is stored as a one-way hash and when you enter you password, it is hashed, and compared to the stored hash.
How to reset the Admin password