Each letter of the series A, D, I, P stands at a place in the alphabet fixed by a rule of its own. Which letter follows P?

The places of the letters in the alphabet are 1, 4, 9 and 16 — the squares of 1, 2, 3 and 4. The next square is 25, and the twenty-fifth letter is Y. Reading the gaps as 3, 5, 7 and then adding 7 again gives W; adding 8 gives X; taking the last letter of the alphabet on the guess that the series must end there gives Z.