1 and 40 share a certain feature, not exactly the same, but closely related.
No other number exhibits this feature.
What is it?
(In reply to re: Possible solution (spoiler)
by Dej Mar)
1. If you want to be so formal , "e" and "i" qualify!. - even in both direction.
2. The category was word problems( words,words,words..) and not math problems.