Subtracting Fractions with Different Denominators

To Subtract Fractions with different denominators:

  • Find the Lowest Common Denominator (LCD) of the fractions
  • Rename the fractions to have the LCD
  • Subtract the numerators of the fractions
  • The difference will be the numerator and the LCD will be the denominator of the answer.
  • Simplify the Fraction

Example: Find the difference between 3/12 and 2/9.

  • Determine the Greatest Common Factor of 12 and 9 which is 3
  • Either multiply the numbers and divide by the GCF (9*12=108, 108/3=36)
  • OR - Divide one of the numbers by the GCF and multiply the answer times the other number (12/3=4, 9*4=36)
  • Rename the fractions to use the Lowest Common Denominator (3/12=9/36, 2/9=8/36)
  • The result is 9/36 - 8/36
  • Subtract the numerators and put the difference over the LCD = 1/36
  • Simplify the fraction if possible. In this case it is not possible

Subtract the Fractions and Reduce to Lowest Terms