Volonter,
document.addEventListener('DOMContentLoaded', function() {
let url_string = window.location.href;
let url = new URL(url_string);
let itemID = url.searchParams.get("pr_id");
let hidden = document.querySelector('.red > .hidden');
hidden?.classList.remove('hidden');
let index = Math.ceil(itemID / 15) * 15 - 1; // let index = Math.ceil((itemID - 1)/ 15) * 15 - 1;
document.querySelector('.red > div')[index]?.classList.add('hidden');
})