Complete the number series: 7, 6, 5, 9, 3, 12, 1, 15, _______?

The correct answer is -1. This series consists of two interleaved sequences:

• Odd positions (1st, 3rd, 5th, 7th, 9th): 7, 5, 3, 1, ? → Each term decreases by 2. So the next odd‑position term is 1 − 2 = -1.
• Even positions (2nd, 4th, 6th, 8th): 6, 9, 12, 15 → Each term increases by 3.

Since the missing term is at the 9th position (odd), it belongs to the first sequence and follows the subtract‑2 pattern.

Options like 21, -3, and 13 do not follow this interleaved logic, so the only correct answer is -1.