<input type = "number" name = "s" \>
<input type="checkbox" name = "protsent" id = "protsent"/>

<script>
function checkAddress2(num){
var s = document.getElementById('s');
var protsent = document.getElementById('protsent');
if (protsent.checked){
s.value = s.value + (s.value * 10 /100);
}
else{
s.value  = ?;
}
}
</script>