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Let the strip of mat showing around the picture be x inches wide.
Now the width of the picture is 20 inches. Mat strip x inches wide will be seen on BOTH sides of the picture. So the width of the mat will be
20 + x + x = 20 + 2x
Similarly the height/length of the picture is 15 inches. Mat strip x inches wide is seen on the top as well as bottom of the picture. So the total height/length of the mat is 15 + x + x = 15 + 2x
Now perimeter of the mat is 120 inches = 2 times (width + height)
2 (20 + 2x + 15 + 2x) = 120
35 + 4x = 60
4x = 25
x = 6.25 inches
Width of the mat is 6.25 inches
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