border of table and page margin

Dear Gurus,

I have a question on how to set the width of each column, so as the border of table and page margin.

Say in a page with 30 cm width, margin on both sides are 2 cm.

So there's 28 cm in the middle as "available space"

In a table with 4 columns, with each column width is 5 cm.

So my table width is 20 cm.

If I want to expand my table width to 28 cm, with evenly distributed the "width-gained" to each column.

Then each column will be 7 cm.

What can I do except manually adjust each column width?

Thank you.