Question

A letter lock consists of 4 rings, each ring contains 9 non-zero digits. This lock can be opened by setting a 4 digit code with the proper combination of each of the 4 rings.Maximum how many codes can be formed to open the lock?

Answer 1

Given that the lock has $4$ rings each ring contains $9$ digits.

To form a code we can use any of the $9$ digits on each ring i.e. we have $9$ choices for each ring.

$\therefore$ Total Possibilities $=9\times 9\times 9\times 9$

$=9^4.$

Hence, the answer is $9^4.$