Sure, the true width of a div is calculated as width + padding.
the contentWrapper is set to:
width: 780px;
padding: 0
rightColumn1 is set to:
padding: 10px 10px 10px 15px; //short hand for: top right bottom left
content is set to:
margin: 0 260px 0 0; //short hand for: top right bottom left - this gives the right column 260 px on the right hand side of the content wrapper.
780 - 260 = 520
we need to add the left (10px) and right (15px) padding from the rightColumn1 though
260 + 10 + 15 = 285 This is the true width of the right column
780 - 285 = 495