Insert the missing number 7, 26, 63, 124, 215, 342, (....)
Answer: B
Numbers are (2⊃3; - 1), (3⊃3; - 1), (4⊃3; - 1), (5⊃3; - 1), (6⊃3; - 1), (7⊃3; - 1) etc.
So, the next number is (8⊃3; - 1) = (512 - 1) = 511.