Classifying a group into two different ways

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The discussion revolves around understanding a math problem from an SAT study guide regarding the classification of students. It clarifies that the ratio of seniors to juniors is 4:5, resulting in 4/9 of the group being seniors and 5/9 being juniors. The confusion about why A + B equals 4/9 is addressed by explaining that this represents the fraction of seniors in a group of 9 students. The conversation emphasizes the importance of finding complementary fractions that sum to a whole number. Overall, the focus is on interpreting ratios and fractions in the context of group classifications.
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I don't understand this example. It's from an SAT math study guide. I understand that to find the fraction of the group that is both girls and seniors, 2/3 is multiplied times 2/5. Why is A + B equal to 4/9? Same with A + C.
 
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Why is A + B equal to 4/9?

As the problem tells you, the ratio of seniors to juniors is 4:5. If there are 9 students, then 4/9 are seniors and 5/9 are juniors.
 
Thanks. I had thought of that but I wasn't sure about it. So I guess the idea here is to, when given a fraction for a group, find the fraction that when added would make the sum of the two a whole number. Fractions are decimal numbers, after all. Percentages come to mind as well.
 

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