JSFiddle - React, Tailwind, and code Playground

HTML

<form name="form" method="post" >
    Quantity:<br />
   <input name="quantity" onfocus="UpdateQuantity();" onkeydown="UpdateQuantity();" type="text" />
   <br />
   Cost: <input name="total" type="text" value="0" size="5" />
 </form>

JavaScript

function UpdateQuantity()  {
       this.form.total.value = this.form.quantity * 45;
    }
    document.onkeydown=UpdateQuantity;