A series runs backwards through the alphabet as Z, T, O, K, H. What is its next letter?

The series runs backwards through the alphabet by a step that shrinks each time: Z to T is six places back, T to O five, O to K four, K to H three, so the next step is two. H stands at place 8, and 8 − 2 = 6, which is F. Taking the next step as one gives G; repeating the step of three gives E; carrying on with a step of five, as though the steps grew rather than shrank, gives C.