Multiple choice general knowledge math & puzzles Which number should come next in the series 1, 3, 6, 10, 15, 8 11 24 21 Reveal answer Fill a bubble to check yourself D Correct answer Explanation The series increases by successive integers: 1(+2)=3, 3(+3)=6, 6(+4)=10, 10(+5)=15. The next increment is +6, so 15 + 6 = 21.