JSFiddle - React, Tailwind, and code Playground

HTML

<div class="summa">400</div>
<div class="summa">500</div>
<div class="summa">600</div>
<div class="summa">700</div>

JavaScript

$(function() {
		let percent = 20; // Скидка в процентах
    $('.summa').each((key, val) => {   
    	$(val).text($(val).text()	- $(val).text() * percent * 0.01);	
    });
});