Subtracting Mixed Fractions

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When subtracting mixed fractions, simplify the process by finding the least common denominator. Learn how to borrow from the whole number when subtracting mixed fractions with help from a math teacher in this free video on fractions in math.

Part of the Video Series: Fractions & Proportions
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Video Transcript

So how does one subtract mixed fractions? Hi, I'm Jimmy Chang. I've been teaching College Mathematics for nine years and subtracting fractions can be a little difficult especially when it comes to the mixed number; well, mixed fractions rather. But once you understand a couple of adjustments, then, subtracting them is going to be pretty straightforward as long as you know what those adjustments are. So here's an example, let's do what you may have to do. Suppose you have a problem of the magnitude, 3 1/2 - 1 3/4. Now right off the bat did you know that the common denominators are not there. So what you have to do is find the least common denominator. The least common denominator between 2 and 4 is 4. Now because the bottom denominator never really changes, this one stays the same. But because you have to know what number 2 has to be multiplied by to give you 4, you know that you have to multiply the top and bottom by 2. So that means you have 3 and 2/4. Normally, you would the subtraction now. But as you can see, 2/4 does not subtract by 3/4. The bottom number is bigger than the top number. So what you need to do is you need to borrow from the 3. Whenever that happens, you must borrow. So, you need to borrow 1 whole from 3. The 3, because it's being borrowed, now turns into a 2. Now this is always going to occur when you have to borrow. The number of pieces that you're borrowing is always going to equal to what the denominator is. In other words, the 2 is now adding 4 more to the numerator. So as a result, this new fraction that we have is going to be 2 and 2 4; 6/4 and you're subtracting 1 3/4. Because the top numerator is now bigger than the bottom number, then you can subtract; 6/4 - 3/4; that's going to give you 3/4. And then once you subtract the fractions, you can subtract the whole numbers; 2 - 1 which is equal to 1. So in the end, 3 and 1/2 - 1 and 3/4 is going to give you 1 and 3/4. So at times you may need to borrow from the whole number; but if that, it makes life that much easier. So I'm Jimmy Chang and that's a demonstration on how you subtract mixed fractions.

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