An old billionaire is writing his will, and deciding how to distribute his nine estates among his three children. He wants to give each chil

An old billionaire is writing his will, and deciding how to distribute his nine estates among his three children. He wants to give each child at least one estate, and he does not want to give any two of his children the same number of estates. In how many possible ways can he distribute his estates? 

 

I HATE CASEWORK!!!!! plz help