Question

A letter lock consists of three rings each marked with 10 different letters. In how many ways it is possible to make an unsuccessful attempt to open the lock?

Answer 1

Number of ways of marking each of the ring = 10 different letters
∴ Total number of ways of marking any letter on these three rings = 10$\times$10$\times$10 = 1000
Out of these 1000 combinations of the lock, 1 combination will be successful.
∴ Total number of unsuccessful attempts = 1000 $-$1 = 999