Q. Find Common Denominators?

What does it mean?

Say you have two fractions, like

1/3 and 8/21

And they have different denominators (3 and 21). Sometimes, you'd prefer that the denominators be the same. It's possible to rewrite these fractions so that the denominators are the same-this is called finding common denominators. Read on to find out how!

2. How do you do it?

Let's use these two fractions for an example:

1/3 and 8/21

Step 1. Factor the denominators.

1/3 and 8/3.7

Step 2. Notice what the differences are. The one on the left had just a single factor, 3. The one on the right has two factors, 3 and 7.

Step 3. Figure out what extra factors are needed to make the denominators equal. If the left-hand denominator had a factor of 7, then they would be equal.

Step 4. Put in the extra factors. (Be sure to put them both in the numerator and denominator!) Here, we'll put the extra 7 factor in the left hand fraction, both top and bottom:

1.7/3.7 and 8/ 3.7

(Tip: It is important to realize that multiplying both the top and bottom by 7 doesn't change the value of the fraction.

You're done! The denominators are the same! You can multiply out if you want:

7/21 and 8/21