Etymon
pie-penkwe-protogermanic-fimf-protoindoeuropean-dekm-protogermanic-tehun-protogermanic-fimftehun-anglian-old-english-fiftiene-middle-english-fiftene-english-fifteen-pregreekder
A dedicated page for this root is on the way — it will gather every word descended from it, sibling forms across daughter languages, and a branching map of the family tree.