Solution
For XYZ
[tex]\begin{gathered} (7\times10\times10)\text{ = 700} \\ 700-1=699\text{ways} \\ \end{gathered}[/tex]For MN
[tex]\begin{gathered} (10\times10)\text{ =100} \\ 100-1=99\text{ways} \end{gathered}[/tex]For ABCD
[tex]\begin{gathered} (10\times10\times10)\text{ =10,000} \\ 10,000-1\text{ =9,999ways} \end{gathered}[/tex]Finally
[tex]\begin{gathered} (699\times99\times9999)-1 \\ =691,940,798\text{ } \end{gathered}[/tex]Social security numbers are
possible 691, 940,798