Can any one explain this to me?

Question

The formula is Y=(X + 1)/2, where X is the number of cards and Y is the placement of the card in the pile after the 3rd deal. In this case, since X is 21, Y = 22/2 = 11.

Answer 1

i suppose the formula is given.

it means that when you have 21 cards (X being the number of cards), the card you selected prior to dealing is the 11th card in the pile (after the 3rd deal).

Answer 2

OK

Looks right to me.  So you have 21 cards and after the 3rd deal, you are placing the card in the 11th place in the pile.  What is your question?