On Monday, a class has 8 girls and 20 boys. On Tuesday, a certain numb

8+x /20-2x = 7/4
18x=108
x =6
2x=12

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Let g be the number of girls who joined the class on Tuesday. We can create the equation:

(8 + g)/(20 - 2g) = 7/4

4(8 + g) = 7(20 - 2g)

32 + 4g = 140 - 14g

18g = 108

g = 6

So 6 x 2 = 12 boys left the class.

Answer: D
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Why we are subtracting 2g from boys, as it was the boys who left. So shouldn't it be (8+2b)/(20-b)?

Response:

The exact sentence describing the relation between the number of boys who left and the number of girls who joined is “a certain number of girls joined the class just as twice that number of boys left the class.” I think you interpreted this sentence as “a certain number of girls joined the class and this number is twice the number of boys who left,” but it is not true. From this sentence, we can deduce the following: 1) a certain number of girls (g) joined the class, 2) twice that number of boys (2g) left the class. That’s why we add g to the number of girls and subtract 2g from the number of boys.
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