Number of options on 1st ring = 4 Number of options on 2nd ring = 4 Number of options on 3rd ring = 4 Number of options on 4rd ring = 4 Number of options on 5rd ring = 4 ∴ Total number of ways of all the options = 4 × 4 × 4 × 4 × 4 = 1024 ∴ Number of all possible unsuccesful attempt to open the lock is =1024−1=1023