Which pair of ratios forms a proportion ?

Let's go through each answer choice

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.

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.

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.

Choice D
9/16 = 3/8
9*8 = 16*3 ... cross multiply
72 = 48
Similar to A and C, we can also eliminate D



So once again the final answer is choice B. 

38+105= 143, 39+104=143, and you will find then adding the numbers on opposite sides, moving inwards yields the answer 138 every time. because of this, you’re going to multiply 138 times the number of pairs, which is 34. therefore, 34*138= 4692(i’m sorry if it’s the wrong numbers, but it’s the right concept)