Wic_,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
.txt{
min-width:200px; float:left; width:50%; height:200px;
border: 1px solid red;
box-sizing: border-box;
}
@media screen and (max-width: 680px) {
.txt {width: 100%;}
}
</style>
</head>
<body>
<div class="txt">Text</div>
<div class="txt">Text</div>
</body>
</html>