When working on the layout of a web page, it helps to have a formula for calculating the width setting you should apply to boxes inside of the container div. This formula takes into account all padding and margins that might have an effect on the dimensions. Here is the formula:
width of inner boxes = width of container - left padding of container - right padding of container - (number of inner boxes * left margin of inner boxes) - (number of inner boxes * right margin of inner boxes) - (2 * number of inner boxes * border width on inner boxes)