Let's go through each answer choice
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Choice A
5/12 = 7/15
5*15 = 12*7 ... cross multiply
75 = 84
The last equation is false so the first equation is false. This means choice A is NOT a proper proportion. We can eliminate choice A.
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Choice B
12/16 = 15/20
12*20 = 16*15 ... cross multiply
240 = 240
We have a true equation at the end (since we have the same value on both sides) so the first equation is true. This means choice B is a proper proportion. Note: both reduce to 3/4 which is 0.75 in decimal form. You can use a calculator to confirm.
We can stop here since we have the answer. However, for the sake of completeness, let's check the other answer choices.
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Choice C
8/32 = 1/3
8*3 = 32*1 ... cross multiply
24 = 32
We can eliminate choice C for similar reasons as done in choice A.
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Choice D
9/16 = 3/8
9*8 = 16*3 ... cross multiply
72 = 48
Similar to A and C, we can also eliminate D
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So once again the final answer is choice B.