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What is the theory called when you are adding simple fractions, i.e. 3/4 + 2/5 = and you just multiply the denominators (4x5 =20) and for the numerator answer, you just cross multiply and add (3x5 + 2x4), getting 15+8=23? The answer will be 23/20, which at that point you reduce. Why does this work? I saw someone doing it. It worked quite well, really quick, and I want to explain it to students.
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