Youngster absorbing one round number — who benefits?

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
a Latin phrase from Cicero. It means "to whom for a benefit," or "who profits by it?" not "to what good purpose?" as is often erroneously claimed. From cui "to? for whom?," an old form preserved here in the dative form of the interrogative pronoun quis ...

Wiktionary Word definitions in Wiktionary
n. The principle that the ultimate initiator of an action is likely he who stands to gain from the action.

Wikipedia Word definitions in Wikipedia
Cui bono , literally "to whose profit?", is a Latin phrase which is still in use as a key forensic question in legal and police investigation: finding out who has a motive for a crime. It is an adage that is used either to suggest a hidden motive or to ...

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Cui bono \Cui` bo"no\ [L.] Lit., for whose benefit; incorrectly understood, it came to be used in the sense, of what good or use; and hence, (what) purpose; object; specif., the ultimate object of life.