3, 7, 15, 31, ?, 127

3, 7, 15, 31, ?, 127

Explanation

3 × 2 + 1 = 7

7 × 2 + 1 = 15

15 × 2 + 1 = 31

31 × 2 + 1 = 63

63 × 2 + 1 = 127