How do you subtract fractions with un like denominators Solution get the common denominator of the fraction. for example, 4/5 - 3/4 = 1/20 ( 5 and 4 \'s least common denominator is 20) * 20 divided by 5 times 4 is 16 * 20 divided by 4 times 3 is 15 *16/20 -15/20 = 1/20.