I'm not going through the math because several already have. The correct answer is 90 because the numbers 2, 3, 4, 5, and 6 in the problem constitutes numbers in a sequence to infinity with the answer being where number 9 is within the sequence. For the answer to be anything else, then 2, 3, 4, 5, and 6 would have to all be random numbers, which they are not and there is nothing in the problem to indicate the numbers are random.