If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

### Unit 4: Lesson 1

Strategies for adding and subtracting fractions with unlike denominators

To add two fractions with different denominators, you need to find the least common multiple of the denominators. You can then rewrite both fractions with this common denominator, which will allow you to add the numerators together. In the example given, 5/6 and 1/4 are rewritten as 10/12 and 3/12, respectively, resulting in a sum of 13/12.

## Want to join the conversation?

• • So you basically change both of the fraction's denominators and change them to something like 3/8+4/6 the denominator would be 24? • • Example: 3/4 is equal to 6/8. They are equal because 3 x 2=6 and 4 x 2=8. Another Example: 2/4 is equal to 1/2 because 2 divided by 2 =1 and 4 divided by 2 =2. as long as you multiply or divide both numbers by the same thing, they are equal. The same applies for mixed numbers and improper fractions. Hope that was helpful!
• • • • The first thing you would have to do is to change all of the denominators to a common denominator. To do this, find the lowest number that is divisible by all of your denominators. In this case, that number would be 100.

Then look at the numerator. To make sure that the fractions are still the same value, we need to change the numerator by the same amount we changed the denominator. So:

4 goes into 100 25 times. That means we need to also multiply the numerator by 25. That means that we now have 25/100 instead of 1/4. That means that 1/4 and 25/100 are the same value. We now do this for the other fractions.

5 goes into 100 20 times. So in this case we multiply the numerator by 20. 20 multiplied by 3 is 60. So instead of 3/5 we now have 60/100.

Finally, you need to do the last fraction. 10 goes into 100 10 times. So we need to do 3 multiplied by 10. Then we would get 30/100.

The last thing you have to do is add the numerators back together. Our denominators are now the same, so the new problem is this:

25/100 + 60/100 +30/100

Then we add the numerators, so we do 25+60+30. This comes out to be 115.

So our answer would be 115/100. But this is an improper fraction, so we could also change it into 1 and 15/100, because 100/100 is equal to one.

To simplify it, then we change it to 3/20 by dividing both 15 and 100 by 5.

Our final answer would be 1 and 3/20. Hope this helped!
• • When I do the course challenge, I make a start by seeing if the first question is one I can answer, if not then I watch a video on the topic, if the question is one I can answer then I will answer it. After that I continue in the same way.
This is how I do the course challenge, so don't feel like you have to do it that way.

Whether you think it is hard or not, you can do it.

Hope i've given you some info about that.
Science123
• At , why didn't the denominator change? Why didn't it stack up to 24 when 10 and 13 stacked up? I thought that what you do to the top, you must do the same to the bottom. So when you add the numerator, then shouldn't the denominator be changed as well?
OOF • Easy way to do it:

Let's say the problem is... you know what, 5/6 + 1/4.

So. Step one is to find the common denominator, which, in this case, is 12.

next, we see how many times 6 goes into 12, which is 2. now we take the top number, 5, and times it by 2, because if we are multiplying the denominator we have to multiply the numerator as well. so- 5x2 is ten. our new fraction is 10/12.
repeat this with the next fraction!  