If you want to add two fractions together, both fractions must have the same denominator. That's okay if you want to add 1/9 and 4/9 = 5/9. But what if you need to add 1/9 and 4/7? How can you add them? You need to change one or both of those fractions so that they have the same denominator.
How can you get 1/9 and 4/7 to both have the same denominator? You need to find a number that has both 9 and 7 as factors.
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