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counting and probability intermediate

Problem

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?
Solution
For every 3 different digits, there is one corresponding descending number, which is just the digits in descending order. So the answer is the number of combinations of three different digits, which is .
Final answer
120