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How many numbers are there between 100 a...

How many numbers are there between 100 and 1000 consists such that every digit is either 2 or 9?

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Clearly, the numbers between 100 and 1000 are 3-digit numbers. So, we have to form 3-digit numbers by using 2 and 9.
Clearly, the repetition of digits is allowed.
Each one of the unit's, ten's and hundred's places can be filled in 2 ways.
...
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