JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" class="txt_1_0" name="a" placeholder="First number"/>
    <input type="text" class="txt_1_0" name="b"  placeholder="Second number"/>
    <input type="text" id="sum" name="sum"  placeholder="Sum"/>
    <input type="text" class="number" name="number"  placeholder="Number"/>
    <input type="text" class="final" name="final" / placeholder="Final">

JavaScript

$('input[type="text"]').on('keyup',function(){
		var a=parseFloat($('.txt_1_0:first').val())
		var b=parseFloat($('.txt_1_0:last').val())
		if(a && b){$('#sum').val((a+b).toFixed(2))}
		//var number=$('.number').val()
		if(number){
			$('.final').val($('#sum').val()*number)
			}
		})