JSFiddle - React, Tailwind, and code Playground

HTML

<form method="post" enctype="multipart/form-data" role="form" id="TextForm">
	<div class="form-group" style="padding-bottom:30px;" >
		<select class="form-control product" name="product[]" id="product"  style="width: 60%;" onChange="szamolo();">
			<option value="1" id="100" class="12.5" selected="selected">name100</option>
			<option value="2" id="200" class="13">name200</option>
			<option value="4" id="300" class="13">name300</option>
		</select>
        <input style="width: 50%; float:left;" name="unit[]" id="unit" type="text"  value="0"  class="form-control units" onChange="szamolo();"/>
	</div>
	
	<div class="form-group" style="padding-bottom:30px;" >
		<select class="form-control product" name="product[]" id="product"  style="width: 60%;" onChange="szamolo();">
			<option value="1" id="100" class="12.5" selected="selected">name100</option>
			<option value="2" id="200" class="13">name200</option>
			<option value="4" id="300" class="13">name300</option>
		</select>
        <input style="width: 50%; float:left;" name="unit[]" id="unit" type="text"  value="0"  class="form-control units" onChange="szamolo();"/>
	</div>
</form>

JavaScript

function szamolo() {
	var price = 0,
	    units = 0,
	    vat = 0,
	    id = 0,
	    teszt = 0,
		szorzat;

	$('.product').each(function() {
			id += parseFloat( $(this).val() );
			price += parseFloat($(this).find('option:selected').attr("id"));
			vat += parseFloat($(this).find('option:selected').attr('class'));
			//teszt +=  parseFloat ($('.product').find('option:selected').attr("id"));
		}
	);
	
	$('.units').each(function() {
			units += parseFloat( $(this).val() );
		}
	);
	
	szorzat = price * units;  
	
	console.log(id+': '+price + ' * '+units+ ' = '+szorzat+' | Vat:'+vat+' | teszt:'+teszt);
	console.log('Szorzat:', szorzat);
};