I heard this number sequence problem about 25 years ago from a maths lecturer.
What number should go in the place of the question mark in this mathematical sequence:
..., 30, ?, 60, 90, 140, 225, 372, 630, ...
I generally dislike number sequence puzzles, but the (intended) answer to this puzzle is so surprising that I feel it should be more widely known. I don't know the origin of this puzzle, so it would be great if anyone has any info on it.
If nobody gets anywhere with it, I will start dropping hints in the comments below to get the ball rolling.