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Why doesn't this work? watch

1. My brother had a question in his homework which was 7/3 + 11/2 and his answer was 18/5, his method was to add the numerators and the denominators and that will get the correct answer. I explained to him that you have to get the same denominator and then add the numerators, but he constantly is asking me why his method is wrong. What can I say for him to understand?
2. The denominators need to be the same or else it won't work. No better way of explaining it, really.
3. Think about a simpler example: 1/2 + 1/3.

Does this equal (1+1)/(2+3) = 2/5?

We note that 2/5 is less than 1/2, but 1/2+1/3 must be greater than 1/2. So the method does not work.

You might like to cut out some paper shapes to illustrate the point visually.
4. Wait - why is this in post-grad applications? ._.

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Updated: January 25, 2016
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