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. $4^9$
• B. ${}^9{P}_4$
• C. $9^4$
• D. none of these

Answer 1

The correct option is C $9^4$

$9\times 9\times 9\times 9\times =9^4$.