A letter has been struck off the head of the series, leaving ?, R, N, K, I, H. Which letter belongs in the empty place?

Read the series from its right end and the rule shows itself: H 8, I 9, K 11, N 14, R 18 rise by 1, 2, 3 and 4 places, so the term before R rises by 5, giving 18 + 5 = 23, which is W. Read the other way, the falls are 5, 4, 3, 2, 1. Taking the first fall as four gives V, as three gives U, and as six gives X.