[ksa]

Сделал так

Вариант без цикла...

<form id="mainForm" name="mainForm" action="#">
  <input id=gr1 type="radio" name="met" value="1">G-1
  <br>
  <input id=gr2 type="radio" name="met" value="2">G-2
  <br>
  <input id=gr3 type="radio" name="met" value="3">G-3
  <br>
  <input id=gr4 type="radio" name="met" value="4">G-4
  <br>
</form>
<script>
document.getElementById("mainForm").addEventListener("click", function () {
	var mtd = document.querySelector('[name="met"]:checked');
	if (mtd) {
		alert("mtd = " + mtd.value);
		var x = +mtd.value + 5;
		alert("x = " + x);
	};
});
</script>