Strategy for Finding Common Denominators

Are you looking for an easy strategy for finding common denominators?

Have you ever given a fraction addition problem and gotten an answer similar to this?  

I know I have. It was always so frustrating. Why do my students keep doing that? Why do they not remember that they need to find a common denominator first?!?! It was always so frustrating to me. Until I started introducing common denominators in a different way. This easy strategy for common denominators will help students remember the importance of them.

I go through these three examples with my students. I ask them what is 2 cats + 3 cats. Their first response is usually “why are you asking me this?” and then when I tell them they will understand in a minute, they answer 5 cats. Then we do the hippo example. Then we do 3 kids + 2 adults. The most common answers are 5 people or 5 humans. I have had other answers, and we talk about how there is more than one right answer for this. We then discuss what is the same and different about the different problems. Then we relate it to fractions.

If we have 2/3 + 3/5, my units are thirds and fifths. Like kids and adults, they are not the same unit. So we have to find something that they have in common. With kids and adults, they are both people. With thirds and fifths, I can make them both into fifteenths. Just like the example with words, I am finding something that both units have in common.

I keep doing this. Throughout the whole unit, at the end of the unit, and after the unit is over as well. I even have kids make up their own examples of real-world problems when they need to change the unit. Those are always hilarious, and having kids come up with their own really help the concept stick.

The first year I introduced common denominators this way was the first year that I gave a problem like the one at the top of this post, and no one, and I mean no one, just added straight across. It stuck with them.They understood why they needed to find a common denominator. It was a breakthrough in my teaching career. I continue to teach this every year, and every year my students continue to be successful!

I hope this quick and easy strategy for finding common denominators is something that you are able to implement in your classroom as well!