This answer is incorrect.
In a proportion you must arrange the quantities so that the same types of quantities appear in the numerators and denominators of both sides of the proportion. So if you are going to put the number of boys in the numerator and the number of girls in the denominator of the left side of the proportion, then you must also put boys in the numerator and girls in the denominator of the right side of the proportion. In this case you know that there are 5 boys to 8 girls. This is the left side of the proportion, with boys in the numerator and girls in the denominator. You also know that there are 40 girls in the class. The number of girls in the class has to be the denominator of the right side of the proportion, leaving the numerator of the right side of the proportion as the unknown (b).
Now let's try another one.
If the ratio of planes to cars is 12 to 3 and there are 8 planes, which proportion should you use to find out the number of cars?
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