JSFiddle - React, Tailwind, and code Playground

HTML

<script>
function saberi(){
var a,b,c,d,e,f;
a=Number(document.getElementById("prvi").value);
b=Number(document.getElementById("drugi").value);
c=Number(document.getElementById("treci").value);
d=Number(document.getElementById("cetvrti").value);
e=Number(document.getElementById("peti").value);
f=Number(document.getElementById("sesti").value);
if(document.getElementById('choice').value == "1,2,5") {
     alert(a+b+e);
}
else if(document.getElementById('choice').value == "1,3,5") {
     alert(a+c+e);
}
else if(document.getElementById('choice').value == "2,3,4") {
     alert(b+c+d);
}
else {
     alert(c+f);
}

}
</script>
<select id="choice">
		<option value="1,2,5" selected>Izaberi prvi, drugi i peti red</option>
		<option value="1,3,5">Izaberi prvi, treći i peti red</option>
		<option value="2,3,4">Izaberi drugi, treći i četvrti red</option>
		<option value="3,6">Izaberi treći i šesti red</option>
	</select>
	<table>
		<tr>
			<td>Prvi red</td>
			<td><input type="text" id="prvi" size="4"></td>
		<tr>
			<td>Drugi red</td>
			<td><input type="text" id="drugi" size="4"></td>
		<tr>
			<td>Treći red</td>
			<td><input type="text" id="treci" size="4"></td>
		<tr>
			<td>Četvrti red</td>
			<td><input type="text" id="cetvrti" size="4"></td>
		<tr>
			<td>Peti red</td>
			<td><input type="text" id="peti" size="4"></td>
		<tr>
			<td>Šesti red</td>
			<td><input type="text" id="sesti" size="4"></td>
		</tr>
	</table>
	<input type="button" value="Računaj" onclick="saberi()">

JavaScript

$tableRows = $("table tr");
$tableInputs = $("table input[type=text]");
$selectChoice =  $("#choice");

var defaultVal = $selectChoice.val();
render(defaultVal);

$selectChoice.change(function()		{
			var val = $(this).val();
      render(val)
});

function render(val)		{
	$tableRows.hide();
  $tableInputs.val("");		// resetovanje polja
	var showArray = val.split(",");
	for(i=0; i<showArray.length; i++)		{
   $tableRows.eq(showArray[i].trim()-1).show();
   }
}