Complete the series: 9, 12, 11, 14, 13, 16, 15, ____.

Explanation

Pattern: 9 + 3 = 12, 12 - 1 = 11, 11 + 3 = 14, 14 - 1 = 13, 13 + 3 = 16, 16 - 1 = 15. Continuing the same pattern gives 18, 17.