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Adding fractions with unlike denominators

Learn how to add two fractions with different denominators. It can be challenging to combine fractions when the denominators don't match. It is important to find a common denominator. Finally, the resource shows how to find a common multiple of the two denominators in order to convert the fractions so they can be added together.

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  • blobby green style avatar for user Richard Dart
    How do I find the common denominator for 3/4+3/8??
    (70 votes)
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    • duskpin ultimate style avatar for user Andy Liao
      First, you'll want to figure out whether or not the larger denominator (3/8) is divisible by the smaller one (3/4). In this case, the 8 is divisible by 4, (8/4 = 2), so you're going to multiply the smaller one (4) by 2, bringing it to 8. Now, you have to remember, whenever you scale fractions, you have to multiply both, so now it's 6/8+3/8 = 9/8 = 1 1/8. In other cases where the larger denominator isn't divisible by the smaller one, find the LCM (least/smallest common multiple), and scale both fractions so the denominators are equal. Hope this helped!
      (150 votes)
  • leafers sapling style avatar for user Jessica Menne
    Can the Rule of Four be used for what he teaches at ? Or is this something different?
    (28 votes)
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  • duskpin seed style avatar for user SinceIwasalittlepieceofpasta
    At to . Since Sal says that 9/10 and 27/10 are the same number, could you have written only 9/10 or 27/30.
    (18 votes)
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  • piceratops tree style avatar for user SUUPER
    I do not understand what if it is a whole number times a fraction
    (11 votes)
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  • blobby green style avatar for user Mylasia608
    How do you make fractions into decimals.
    (12 votes)
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    • winston baby style avatar for user judah.p.greer
      To convert fractions into decimals, divide the denominator into 100, then multiply the answer by the numerator, and finally add the decimal point, i guess. For example, 3/5 is equal to 0.6 because 5 going into 100 is 20 which would make the decimal 0.2. The numerator is 3 and 0.2 x 3 = 0.6. I hope this helped!
      (8 votes)
  • blobby green style avatar for user Rodriguez Jamie Marie
    6 13/21
    + 5 3/7 fraction addition
    (9 votes)
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  • hopper happy style avatar for user Ethan Vu
    what is the best way to multiply fractions
    (0 votes)
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    • leafers tree style avatar for user Sean
      Say you are multiplying 7/8 times 4/9 it would also be written as 7 times 4 which equals 28 and 8 times 9 equals 72 giving you an answer of 28/72. If you didn't understand you multiply the numerator by the numerator and the denominator by the denominator then combine the two to get your answer.
      (28 votes)
  • blobby green style avatar for user TruthbotAI
    Does anyone know why when adding 2 fractions you would multiply each fraction by 1?

    I am reading a book and it does not seem clear to me how or why it works or why you would use it at all rather than the easy cross multiplication method. I just don't want to ignore it just in case there are situations where it needs to be used which I find is always the case that there are rules that work for some combination of things and not others.

    The book I am reading has stumped me in the addition of fractions. Here is a snippet of it:

    1/7 + 3/4

    "Here, we can neither convert sevenths to quarters nor quarters to sevenths, so we’re going to learn a method that you can use to add or subtract any fractions. To get identical bottom numbers, we begin by multiplying each number by 1, written as quarters in one case, and as sevenths in the other."

    1x 1/7 + 1x3/4 = 4/4 x 1/7 + 7/7 x 3/4

    I have no idea how they got the 4/4 and 7/7 try as I might and how the strange-looking order of the terms after the equal sign even if I overlooked how they got those numbers to begin with.

    I thought I worked out how they did it but my brain refused to accept what it thought was either a convoluted approach or that I was creating my own incorrect method lol

    Can anyone provide a possible clarification of why they would suggest using this method rather than the easy cross multiplication method?
    (6 votes)
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    • stelly blue style avatar for user Kim Seidel
      Some basic info you need to know:

      1) The denominator of a fraction tells you the size of each portion. Visualize a pizze that was cut into 8 slice. Each slice is 1/8 of the pizza. If a pizza is cut int 4 slices, then each slice is 1/4 of the pizza and a slice is twice as big as a slice of 1/8

      2) To add & subtract fractions we need to work with fractions of the same size. So, we have to force the denominators to have a common value. This is called a "common denominator".

      3) Equivalent fractions are created by multiplying both numerator & denominator by the same value. Why? This is based upon the identity property of multiplication: Any number times 1 = the original number. So, by multiplying a fraction by 4/4 = 1, we aren't changing the value of the fraction. We're just converting it to an equivalent value.

      Now to your questions - Where did the 4/4 and 7/7 come from?

      Since the fractions in the video don't have a common denominator, they need to be converted to have a common denominator. So, we start by find the lowest common multiple(LCM), also called the lowest common denominator (LCD) of the fractions. The LCM for 4 and 7 is 28. There are lessons on finding an LCM at: https://www.khanacademy.org/math/pre-algebra/pre-algebra-factors-multiples/pre-algebra-lcm/v/least-common-multiple-exercise

      To convert the fraction 1/7 to have a denominator of 28, you ask yourself the question: what times 7 will create 28? The answer is 4. You do the same thing to determine how to convert 3/4 to have a denominator of 28. What times 4 will create a denominator of 28? The answer is 7. These values 4 and 7 become 4/4 and 7/7 because we have to multiply each fraction by a value = 1 to have an equivalent fraction.

      Hope this helps.
      (5 votes)
  • piceratops ultimate style avatar for user @khanacademyuser
    For improper fractions, for example 32/30, does the denominator always have to be equal? If yes, why?
    (3 votes)
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    • piceratops ultimate style avatar for user Hecretary Bird
      When you're adding/subtracting, the denominators have to be equal regardless of whether the fraction is proper or improper. We can easily think of an improper fraction as a mixed number, right? We have a whole number plus a fractional number. For example, if we add two improper fractions that are converted to mixed numbers, it could look like this:
      5/3 + 28/5 = ?
      1 2/3 + 5 3/5 = ?
      (1 + 2/3) + (5 + 3/5) = ?
      Then, using the associative property, we can switch around the parenthesis like this:
      (1 + 5) + (2/3 + 3/5) = ?
      Now you see we have a completely normal unlike denominator problem, and we just add 6 to the answer. As with any unlike denominator adding/subtracting problem, you have to get the fractions to the same denominator. In this case, it would be a denominator of 15.
      = 6 + 10/15 + 9/15
      = 6 + 19/15
      = 6 + 1 + 4/15
      = 7 4/15, or 109/15
      (10 votes)
  • blobby green style avatar for user very ugly person
    why does khan academy suck
    (5 votes)
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Video transcript

- [Voiceover] Let's say that we have the fraction 9/10, and I want to add to that the fraction 1/6. What is this, what is this going to equal? So when you first look at this, you say, "Oh, I have different denominators here. It's not obvious how I add these." And you'd be right and the way to actually move forward is to find a common denominator, to convert both of these fractions into fractions that have a common denominator. So how do you think about a common denominator? Well, a common denominator's gonna have to be a common multiple of these two denominators of 10 and six. So what's a common multiple of 10 and six? And it's usually simplest to find the least common multiple, and a good way of doing that is start with the larger denominator here, 10, and say, okay is 10 divisible by six? No. Okay, now, is 20 divisible by six? No. Is 30 divisible by six? Yes. 30 is divisible by six. So I'm just going through the multiples of 10 and saying, "Well what is the smallest multiple of 10 that is divisible by six?" And that's going to be 30. So I could rewrite both of these fractions as something over 30. So nine over 10. How would I write that as something over 30? Well I multiply the denominator, I'm multiplying the denominator by three. So I've just multiplied the denominator by three. So if I don't want to change the value of the fraction, I have to do the same thing to the numerator. I have to multiply that by three as well because now I'm just multiplying the numerator by three and the denominator by three, and that doesn't change the value of the fraction. So nine times three is 27. So once again, 9/10 and 27/30 represent the same number. I've just written it now with a denominator of 30, and that's useful because I can also write 1/6 with a denominator of 30. Let's do that. So 1/6 is what over 30? I encourage you to pause the video and try to think about it. So what did we do go from six to 30? We had to multiply by five. So if we multiply the denominator by five, we have to multiply the numerator by five as well, so one times five, one times five is five. So 9/10 is the same thing as 27/30, and 1/6 is the same thing as 5/30. And now we can add, now we can add and it's fairly straightforward. We have a certain number of 30ths, added to another number of 30ths, so 27/30 + 5/30, well that's going to be 27, that's going to be 27 plus five, plus five, plus 5/30, plus 5/30, which of course going to be equal to 32/30. 32 over 30, and if we want, we could try to reduce this fraction. We have a common factor of 32 and 30, they're both divisible by two. So if we divide the numerator and the denominator by two, numerator divided by two is 16, denominator divided by two is 15. So, this is the same thing as 16/15, and if I wanted to write this as a mixed number, 15 goes into 16 one time with a remainder one. So this is the same thing as 1 1/15. Let's do another example. Let's say that we wanted to add, we wanted to add 1/2 to to 11/12, to 11 over 12. And I encourage you to pause the video and see if you could work this out. Well like we saw before, we wanna find a common denominator. If these had the same denominator, we could just add them immediately, but we wanna find a common denominator because right now they're not the same. Well what we wanna find is a multiple, a common multiple of two and 12, and ideally we'll find the lowest common multiple of two and 12, and just like we did before, let's start with the larger of the two numbers, 12. Now we could just say well 12 times one is 12, so that we could view that as the lowest multiple of 12. And is that divisible by two? Yeah, sure. 12 is divisible by two. So 12 is actually the least common multiple of two and 12, so we could write both of these fractions as something over 12. So 1/2 is what over 12? Well to go from two to 12, you multiply by six, so we'll also multiply the numerator by six. Now we see 1/2, and 6/12, these are the same thing. One is half of two, six is half of 12. And how would we write 11/12 as something over 12? Well it's already written as something over 12, 11/12 already has 12 in the denominator, so we don't have to change that. 11/12, and now we're ready to add. So this is going to be equal to six, this is going to be equal to six plus 11, six plus 11 over 12. Over 12. We have 6/12 plus 11/12, it's gonna be six plus 11 over 12, which is equal to, six plus 11 is 17/12. If we wanted to write it as a mixed number, that is what, 12 goes into 17 one time with a remainder of five, so 1 5/12. Let's do one more of these. This is strangely fun. Alright. Let's say that we wanted to add, We're gonna add 3/4 to, we're gonna add 3/4 to 1/5. To one over five. What is this going to be? And once again, pause the video and see if you could work it out. Well we have different denominators here, and we wanna find, we wanna rewrite these so they have the same denominators, so we have to find a common multiple, ideally the least common multiple. So what's the least common multiple of four and five? Well let's start with the larger number, and let's look at its multiples and keep increasing them until we get one that's divisible by four. So five is not divisible by four. 10 is not divisible by four, or perfectly divisible by four is what we care about. 15 is not perfectly divisible by four. 20 is divisible by four, in fact, that is five times four. That is 20. So what we could do is, we could write both of these fractions as having 20 in the denominator, or 20 as the denominator. So we could write 3/4 is something over 20. So to go from four to 20 in the denominator, we multiplied by five. So we also do that to the numerator. We multiply by three times five to get 15. All I did to go from four to 20, multiplied by five. So I have to do the same thing to the numerator, three times five is 15. 3/4 is the same thing as 15/20, and over here. 1/5. What is that over 20? Well to go from five to 20, you have to multiply by four. So we have to do the same thing to the numerator. I have to multiply this numerator times four to get 4/20. So now I've rewritten this instead of 3/4 plus 1/5, it's now written as 15/20 plus 4/20. And what is that going to be? Well that's going to be 15 plus four is 19/20. 19/20, and we're done.