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Printjmc
counting and probability senior
Problem
For how many three-element sets (order doesn't matter) of positive integers is it true that ? (The positive integers and are distinct.)
Solution
The prime factorization of is Therefore, we have the equation where must be distinct positive integers and order does not matter. There are ways to assign each prime number on the right-hand side to one of the variables which gives solutions for However, three of these solutions have two s and one which contradicts the fact that must be distinct. Because each prime factor appears only once, all other solutions have distinct. Correcting for this, we get ordered triples where are all distinct.
Finally, since order does not matter, we must divide by the number of ways to order This gives the final answer,
Finally, since order does not matter, we must divide by the number of ways to order This gives the final answer,
Final answer
40