What should come in place of the question mark (?) in the given series? 40, ? 18 10 4 0

The differences between consecutive terms are: 40–?=12, ?–18=10, 18–10=8, 10–4=6, 4–0=4. The differences form a decreasing sequence: 12, 10, 8, 6, 4 (decreasing by 2). So ? = 40 – 12 = 28.