[рони]

sergylt,

<form name="form" action="http://">
<input class="chek" type="checkbox">
<input class="chek" type="checkbox">
<input class="chek" type="checkbox">
<input class="chek" type="checkbox">
<input class="chek" type="checkbox">
<input class="chek" type="checkbox">
<input class="chek" type="checkbox">
<input class="chek" type="checkbox">
<input class="chek" type="checkbox">
<div  class="rezultat"></div>
</form>

<script>
window.addEventListener("DOMContentLoaded", function() {
    var a = document.querySelector("form"),
        b = document.querySelector(".rezultat");
    a.addEventListener("change", function() {
        b.innerHTML = "Total: " + [].reduce.call(a.querySelectorAll(":checked"), function(a, b, c) {
            return a + (5 > c ? 10 : 5)
        }, 0)
    })
});
</script>