To get the width of its border we must first find the area of the border.
Total Area - Area of photo = Area of border
280 - (12)(18) = Area of border
Area of border = 64 sq cm
We now have a rectangle with a hole of 12x18 , so now we cut the area of the border such that we will have 2 (18xW), 2 (12xW), and 4(WxW) squares.
by this info we will have an equation
64= 2(18xW) + 2(12xW) + 4(WxW)
simply
4W^2 + 60W = 64
We then solve for W
and it would be
W = 1 cm Hope my answer would be a great help for you.
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