ParasomNO

ParasomNO.user.js

// ==UserScript==
// @name        ParasomNO
// @namespace   parasomno
// @include     https://www.amazon.co.uk/gp/buy/spc/handlers/display.html*
// @version     1
// @grant       none
// @require     https://ajax.googleapis.com/ajax/libs/jquery/2.2.0/jquery.min.js
// ==/UserScript==

function getRandomInt(min, max) {
    return Math.floor(Math.random() * (max - min + 1)) + min;
}

$(".place-order-button-link").hide();

var first = getRandomInt(0, 50);
var second = getRandomInt(0, 50);

$('<p>' + first + ' plus ' + second + '</p>').insertBefore(".place-order-button-link");
$('<input type="text" id="parasomno-answer" />').insertBefore(".place-order-button-link");
$('<a href="#" id="parasomno-submit">Submit</a>').insertBefore(".place-order-button-link");

$('#parasomno-submit').click(function(event) {
  if (first+second == $('#parasomno-answer').val()) {
   $(".place-order-button-link").show();
  }
  even.preventDefault();
});