Answer: Choice A) 15 feet
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x = length of fourth side in quadrilateral ABCD
Have a look at the attached image I've posted. I made a chart that lines up the side lengths from largest to smallest. Note how the first two items in the bottom row are blank. We don't know these values and we don't need to find out in order to find x.
Based on the last two columns of that table, we can form the proportion below, which is then used to isolate x.
30/12 = x/6
30*6 = 12*x ... cross multiply
180 = 12*x
12*x = 180
x = 180/12 ... divide both sides by 12
x = 15
(210+200+190+180+300+150+90+x)/8=200
(1320+x)/8=200 /*8
1320 +x=1600
X=1600-1320
X= 280
If 19 is half (50%) of something, find that number you would just have to multiply 19 by 2.
This means that 19 is 50% of 38.