Plural of fifteenth; multiple parts when something is divided into 15 equal pieces, or ordinal positions numbered 15.
From fifteenth (ordinal number from fifteen) plus -s (plural). The ordinal form emerged to describe positions and fractions in mathematical and sequential contexts.
If you divide a pizza into 15 slices (unusual but theoretically possible), each slice is 1/15—though this exact fraction is so impractical that you'd almost never hear 'fifteenths' in real life.
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