first-child

@media (min-width: 500px) and (max-width: 767px){
/* 概要2段組み */
.gaiyou .col    {float: left;
width: 48%;
margin-left: 4%;}
.gaiyou .col:first-child + .col + .col    {clear: both;
margin-left: 0;}
}
.cols .col:first-child    {margin-left: 0;}
.cols:after    {content: "";
display: block;
clear: both;
}