JSFiddle - React, Tailwind, and code Playground

HTML

<form>
<input id="quantity" class="qty" type="text" placeholder="quantity" />
    <input type="button" value="calculate" onclick = "update();" />
</form>

<!--display total cost here--> 
<span id="ans">The Cost is: totalcost</span>

JavaScript

var cost = 25;

function update() {
    var cost = 25;
    var quantity = document.getElementById("quantity").value;
    var totalcost = (cost * parseInt(quantity));
    
    document.getElementById("ans").innerHTML = "The Cost is:" + totalcost;
    
   // send total back to html document and display as total cost
    
}