Read this sentence from the article. Rudolph was already a known quantity in the world of track.
What does the idiom “a known quantity” mean in the sentence above?

a known quantity means, someone or something whose abilities or characteristics are well known. When she was cast in the role of the princess, she was already a known quantity from her work in several films.

so basically Rudolph was known for something either about the character or something the character does. I hope this helps! :)

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