Each number in the series is the preceding number multiplied by 2 and then increased by 1. Thus,(3 x 2) + 1 = 7,(7 x 2) + 1 = 15,(15 x 2) + 1 = 31 and so on. Missing number = (63 x 2) + 1 = 127.

Clearly,5 + 3 = 8,8 + 3 = 11,11 + 3 = 14,…… So,the series is an A.P. in which a = 5 and d = 3. Let the number of terms be n. Then,320 = 5 + (n - 1) x 3 or (n - 1) = 105 or n = 106.