JSFiddle - React, Tailwind, and code Playground

HTML

<span class="b-iter">15</span>

CSS

.b-iter{
  font-size: 35px;
}

JavaScript

var timer,
  num = 15,
  output = $('.b-iter');
  
function changeNum() {
  output.text(num--);
  if (num >= 0) setTimeout(changeNum, 1000 + (15 - num) * 3000);
}
setTimeout(changeNum, 1000);