There are only 26 letters in the English alphabet. The 300th letter doesn't exist. To find which letter it would be if the alphabet repeated, you would divide 300 by 26:
300 ÷ 26 = 11 with a remainder of 14.
This means the alphabet repeats 11 times completely, and then the 14th letter is the 300th letter. The 14th letter of the alphabet is N.