What’s given in the problem  The English alphabet has 26 letters. We consider the 20th letter from the beginning. We consider the 20th letter from the end. Then find the letter in the middle of these two letters. 
Step 1 . Find the 20th letter from the beginning. The 20th letter from the beginning of the English alphabet is T.
Step 2 . Find the 21st letter from the end. The 21st letter from the end of the English alphabet is F.
Step 3 . Find the letter in the middle of T and F. The sequence of letters between T and F is: T, S, R, Q, P, O, N, M, L, K, J, I, H, G, F. There are 15 letters in this sequence. The middle letter is the 15+1 /2=8th letter. The 8th letter in the sequence is M.
Solution
The letter in the middle of the sequence is M.