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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?
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Solution
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
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