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?
Solution:
Number of ways of marking each of the ring = 10 different letters
$\therefore$ 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.
$\therefore$ Total number of unsuccessful attempts $=1000-1=999$
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