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, it is a slight trick.
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
|2009 UNCO Math Contest II (Problems • Answer Key • Resources)|
|1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10|
|All UNCO Math Contest Problems and Solutions|