top of page
Search
• Merry Havens

# Quick rule for adding/subtracting fractions

First Rule: You need a common denominator (CD) when adding/subtracting fractions. If you have a CD, great, just add/subtract the numerators and keep the denominator. For example: 1/4 + 2/4 (CD!) = (1 + 2 = 3) / 4 --> 3/4. If you don't have a CD, that's OK. Here's a "quick fix" :

1/3 + 1/4 (no CD) , so, do this: 1. multiply the 1 (numerator of the first fraction) times the 4 (denominator of the second fraction), you get 4. 2. Multiply the 1 (numerator of the second fraction) times the 3 (denominator of the first fraction), you get 3. 3. Add the 3 + 4 , you get 7, the numerator. 4. Multiply the denominators to get the CD, so, 3 x 4 is 12, the denominator. Now, you have the solution: 7/12

Practice makes confidence, so do that same process on these:

A. 1/2 - 2/5 =

B. 2/3 + 1/5 =

C. 2/7 + 2/5 =

Solutions:

A. (1 x 5) - (2 x 2)/ (2 x 5) = (5 - 4)/ 10 = 1/10

B . (2 x 5) + (1 x 3) / (3 x 5) = ( 10 + 3)/ 15 = 13/15

C. (2 x 5) + (2 x 7)/ (7 x 5) = (10 + 14) / 35 = 24/35