JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" class="form-control amount" value="" placeholder="amount" />
<input type="hidden" class="price" value="3.30" />
<input type="text" class="form-control total" value="" disabled />

JavaScript

$('.amount').on('keyup', function (e) {
	e.preventDefault();
	
	var amount = $('.amount').val(); 
	var total = Number(amount) * Number($('.price').val());
  
  $('.total').val(total);

});