A number is called a "decreasing number" if each digit in the number is less than the digit to its left. For example, 87420 is a decreasing number. How many five-digit decreasing numbers are there?

Answer:   252Step-by-step explanation:The question is fully equivalent to asking how many subsets of length 5 there are of 10 objects (digits 0–9). That number is 10C5, where nCk is the number of ways to choose k objects from a list of 10. The value of that is ...   nCk = n!/(k!(n-k)!)There are 252 ways to choose 5 numbers from the digits 0-9:   10C5 = 10!/(5!(10-5)!) = 10·9·8·7·6/(5·4·3·2·1) = 9·4·7 = 252_____The order of the selection doesn't matter, because the selected digits are always arranged in decreasing order to form a decreasing number.