Let A be a set of n $(\ge 3)$ distinct elements. The number of triplets (x, y, z) of the A elements in which at least two coordinates is equal to 

Total number of triplets without restriction is n x n x n. The number of triplets with all different coordinates is ${ }^{\mathrm{n}}{\mathrm{P}}_3$. Therefore, the required number of triplets is ,${\mathrm{n}}^3-{\mathrm{n}}_{{\mathrm{P}}_3}$