Srinivasa Ramanujan was said to have answered this riddle almost immediately when brought to his attention by Prasanta Chandra Mahalanobis, taken from a December 1914 print of The Strand Magazine:
He said the house of his friend was in a long street, numbered on this side one, two, three, and so on, and that all the numbers on one side of him added up exactly the same as all the numbers on the other side of him. Funny thing that! He said he knew there was more than fifty houses on that side of the street, but not so many as five hundred.
What House number did his friend live in?
Note: the house of his friend is not included in either sum
Bonus question: How can you find all possible solution(s) for any number of houses?
You can read more about the context of this puzzle here here or you can watch the mathologer video Jaap Scherphuis commented below