Multiple choice general knowledge math & puzzles 1, 5, 9, 15 ... what is the Next Nunmber? 17 16 21 19 Reveal answer Fill a bubble to check yourself C Correct answer Explanation The pattern is based on the differences between consecutive numbers: 1 to 5 = 4, 5 to 9 = 4, 9 to 15 = 6. The differences themselves follow a pattern: 4, 4, 6, 6, 8, 8... So the next difference is 6, making 15 + 6 = 21.