<!DOCTYPE html>
<html>
<head>
<title></title>
<style type="text/css">
.block {
width: 190px;
height: 40px;
background-color: red;
border: 1px solid black;
}
</style>
</head>
<body>
<div class="block"></div>
<div class="block"></div>
<div class="block"></div>
<div class="block"></div>
<div class="block"></div>
<div class="block"></div>
<script type="text/javascript">
"use strict";
let blocks = Array.from(document.querySelectorAll(".block"));
let widthBlock = getComputedStyle(blocks[0]).width.slice(0, -2);
for (let i = 0; i < blocks.length; i++) blocks[i].style.width = +widthBlock + 20 * i + "px";
</script>
</body>
</html>