Propositiones ad Acuendos Juvenes/Problems/42 - De Scala Habente Gradus Centum/Historical Note
Historical Note on Propositiones ad Acuendos Juvenes by Alcuin of York: Problem $42$: De Scala Habente Gradus Centum
David Wells calls to mind the apocryphal story (quite possibly untrue) about Carl Friedrich Gauss, where at the age of $8$ he solved the problem of adding the numbers from $1$ to $100$ in a few seconds.
Alcuin's is the earliest text which dresses the problem up in fancy clothes, and many European texts follow his example.
In such renditions, the number of elements to add is almost always $100$.