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MathRevolution
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It will sound weird...
The entire shade region if unfolded will be one long rectangle .
the width =d ,and the lengths = to the remaining of 18d-2d =16d ..
Since it is a rectangle each length is now 8d ,so area = l*b=8d * d =8*20*20
or as in my original post 20*8*20...
I hope I am clear.. I am not so good explaining ...but if you want more explanation I will try
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nikhiljd still not clear :'(
MathRevolution please post the official solution
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Forget conventional ways of solving math questions. In DS, Variable approach is the easiest and quickest way to find the answer without actually solving the problem. Remember equal number of variables and independent equations ensures a solution.

A larger playground has a rectangular-shaped track shaded shown as above figure such that the track has a uniform d as its width, what is the track’s area?

1) The perimeter of smaller playground is 18d.
2) The perimeter of smaller playground is 360.
Attachment:
square.jpg
square.jpg [ 4.53 KiB | Viewed 2382 times ]

Just like the picture above, suppose width and height as a and b respectively. Then, the area of the track is (a+2d)(b+2d)-ab=(a+b)2d+4d^2 and there are 3 varaibles(a,b,c). In order to match with the number of equations, you need 3 equations. For 1) 1 equation, for 2) 1 equation, which is likely to make E the answer.
When 1) & 2), from 2(a+b)=18d=360, a+b=180 and d=2 are derived, which is unique and sufficient.
Thus, the answer is C.
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