Each letter of the English alphabet is replaced by the letter that stands the same number of places from the end of the alphabet as it stands from the beginning — A by Z, B by Y, and so on. Which letter would replace J?

J is the 10th letter from the beginning, so it must be replaced by the 10th letter counted from the end. Counting back Z, Y, X, W, V, U, T, S, R, Q gives Q, which is also 26 − 10 + 1 = the 17th letter from the beginning. P (16th) and R (18th) are the one-place slips on either side, and K is simply the letter next to J.