Explanation

5 × 2 = 10 - 1 = 9

9 × 2 = 18 - 1 = 17

17 × 2 = 34 - 1 = 33

33 × 2 = 66 - 1 = 65

65 × 2 = 130 - 1 = 129

129 × 2 = 258 - 1 = 257