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?

• A. $9^4$
• B. $4^9$
• C. $7^4$
• D. None of these

Answer 1

The correct option is A $9^4$

The question is basically choosing any number in $9$ non-digit number $4$ times where repetition is allowed which is basically =$9\times 9\times 9\times 9=9^4$