.main{width:100%;background-color:gray;padding:5px}
.header{height:40px;background-color:blue;}
.leftnav{float:left;width:50px;height:260px;background-color:red;}
.content{ width: 70%; float:left;margin-left:50px;height:260px;background-color:white}

Please set the width and float in class "content" and set the width in class "main" to 100% or required width in px

Er. Pankaj Sinha