We call a number a descending number if each digit is strictly smaller than the digit that comes before it.  For example, 863 is a descending number.  How many 3-digit descending numbers are there?
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
We call a number a descending number if each digit is strictly smaller than the digit that comes before it.  For example, 863 is a descending number.  How many 3-digit descending numbers are there?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
120
We call a number a descending number if each digit is strictly smaller than the digit that comes before it.  For example, 863 is a descending number.  How many 3-digit descending numbers are there?
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