JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container">
    
 <div class="box"><div class="box_in">Box</div></div>
    
 <div class="box"><div class="box_in">Box</div></div>
    
 <div class="box"><div class="box_in">Box</div></div>
    
 <div class="box"><div class="box_in">Box</div></div>
    
 <div class="box"><div class="box_in">Box</div></div>
</div>

CSS

.box{
    display:block;
    float:left;
   
}
.box_in{
    margin:5px;
    padding:5px;
     background:green;
}

JavaScript

$(document).ready(function(){
        var cw= $('#container').width();
        $(".box").width(cw/5);
}); 
$(document).resize(function(){
        var cw= $('#container').width();
        $(".box").width(cw/5);
});