Question 1146705
<br>
The border is on all 4 sides and is presumably of equal width everywhere.  So each dimension of the mat board is the dimension of the picture plus the width of the border twice.  So the width of the border is half the difference between the width of the picture and the width of the mat board.<br>
width = (1/2)(11 1/2 - 8 3/4)<br>