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known quantity

noun
1.
Mathematics. a quantity whose value is given: in algebra, frequently represented by a letter from the first part of the alphabet, as a, b, or c.
2.
any factor, circumstance, etc., that is already accepted or familiar:
Her honesty is a known quantity.
Origin
1975-1980
1975-80
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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