He has to center a 5-in-wide picture in a 16-in-wide page.
Anywhere he puts the picture, the sum of the space not occupied by the picture will be
When the picture is in the center of the page, both sides have the same empty space, so we have to divide the empty space left by the picture in two equal parts:
The picture, when centered, will have 5.5 inches at each side.
He should put the picture 5.5 inches from each side of the page.