Multiple choice general knowledge math & puzzles A,C,F,J,O.. What is the next in teh series? T U S V Reveal answer Fill a bubble to check yourself B Correct answer Explanation The gaps between letters increase by one each time: A(+2)C, C(+3)F, F(+4)J, J(+5)O. Following this pattern, the next gap should be +6. O is the 15th letter; 15 + 6 = 21. The 21st letter of the alphabet is U.