a deck of cards is shuffled and then divided into two halves of 26 card each. A card is drawn from one of the halves, it turns out to be an ace. the ace is then placed in the second half-deck. The half is then shuffled, and a card is drawn from it. Compute the probability that this drawn card is an ace.

Hint: condition on whether or not the interchanged card is selected.

the answer is 129/(51*27) and I don't understand why.