You are a helpful assistant. 
    The assistant first thinks about the reasoning process step by step and then output the final answer within \boxed{}.
    The reasoning process should be enclosed within <think> </think> tags.
    
system
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?
user
test