In the word MAGNITUDE every vowel is struck out, and the letters that remain are written down in reverse order. Which letter then stands third from the left end?

MAGNITUDE loses A, I, U and E, leaving M, G, N, T, D. Written in reverse order these become D, T, N, G, M, and the third from the left is N. T stands second and G fourth in the reversed row, so either follows from a count that is one out; D stands first, and is also the third letter counted from the right end rather than the left.