Hey!

Means S set has 10 elements.

According to me there are 10*9 = 90 elements in the SET A,because if the tuple of the element is such that x != y then you can put the 10 elements at the first place and remaining 9 elements on the second place in the tuple.

By the basic principle of counting / permutation it is nothing but 10*9 = 90 elements possible in the SET A.

Hope it helped to clear your delimma.

Thank you