Common Denominator

Common Denominator

Multiple of the denominators of two or more fractions.

The lowest common denominator for two or more fractions (LCM) is the lowest whole number that is also a multiple of each denominator in these fractions.


  • Consider the fractions \(\frac{4}{12}\) and \(\frac{8}{18}\) : the numbers 36 and 72 could act as a common denominator, but the lowest option is 36, which we call the Lowest Common Multiple of 12 and 18 and that we write as: LCM (12, 18) = 36.
  • The common denominator of the fractions \(\frac{3}{10}\), \(\frac{7}{15}\) and \(\frac{11}{20}\) is 60.
    Therefore, we write: LCM (10, 15, 20) = 60.

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