Setting div width to 100% minus certain amount of px

Question

Have been searching the net for the past hours to find a solution to this, but couldn\'t find it, so here I am.

I need the width of the div to be

Answer 1

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#left {float: left; width: 390px; background: #f76922;}
#content {overflow: hidden; background: #eee;}

float the left div and make a new block formatting context out of the right div (overflow: hidden is one way to do that), that way it will take the remaining space

Example Fiddle