2009 UNCO Math Contest II Problems/Problem 1
How many positive -digit numbers are there such that For example, and have this property but and do not.
Here, we need to find such that , and where .
If we place , then we can place as , i.e. in ways.
Similarly, if we place , we can place i.e. in ways.
If, we place , we have the only choice , in ways.
So, in order to get the number of possibilities, we have to add the no. of all the possibilities we got, i.e. the answer is
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