[ermak20]

Сделал так

<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>
var form = document.getElementById("mainForm");
var mtd = document.getElementsByName("met");

form.addEventListener("click", function () {
   for (i = 0; i < mtd.length; i++) {
    if (mtd[i].checked) {
      alert("mtd = " + mtd[i].value);
      var x = +mtd[i].value + 5;
      alert("x = " + x);
    }
  }
});
</script>