Video Transcript

Hi my name is Marija and I'm a mathematician and today I'm going to teach you how to add and subtract fractions with unlike denominators by using cross multiplying. So if we've got the fraction one third and we want to go ahead and add it to two fifths. All right so we don't have the same denominators and we know that we need the same denominator before we can add them. One way is to go ahead and look for the least common denominator, but if you're not so great at that or you want an easy way out, we're just going to do it by cross multiplying. So I'm going to do fives times one, which is five. I'm going to bring down my plus sign, and I'm going to do three times two, which is six. And my denominator is going to be the product of the two denominators. So I'm going to do three times five, which is 15. All right I'm going to go ahead and add the numerators, so five plus six is 11. Keep my denominator, obviously, which is 15. So my sum is 11 over 15. OK. By that same token we'll do a subtraction problem. Let's say we had three fifths minus one half. All right so I don't want to do this by looking for a least common denominator, I'm just going to cross multiply. Three times two, sorry two times three is six. Bring down your minus sign. Five times one is five. Your denominator remember is going to be the product of the two denominators, so I have five times two is ten. I'm going to go ahead and subtract the numerators, six minus five is one. Keep my denominator, which is then, and my difference is one over ten. So that is how you use cross multiplication to add or subtract fractions with unlike denominators.