Carry the series C5, E8, H12, L17 one step further. What is its fifth term?

The letters and the numbers run on side by side. The letters C 3, E 5, H 8, L 12 advance by 2, 3 and 4 places, so the next advance is 5 and the letter is 12 + 5 = 17, which is Q. The numbers 5, 8, 12, 17 rise by 3, 4 and 5, so the next rise is 6 and the number is 23. Carrying the number's rise on at 5 gives Q22; advancing the letter by only four gives P23; advancing it by six gives R23.