Question 17710
let the length be x and width be y
then 
x=2y-10.......[1]
2(x+y)=400....[2]
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substituting [1] in [2] we get
2(2y-10+y)=400
(2y-10+y)=200
3y-10=200
3y=210
y=70
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x=2y-10=140-10=130
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Dimensions are: 70 yards,130 yards
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Hope this helps,
Prabhat