вот такая была задумка. Может чего и намудрил, но теперь работает)
<!DOCTYPE html>
<html>
<head>
<title>Селекты</title>
<meta charset="utf-8">
</head>
<script type="text/javascript">
function calculat (obj) {
var num55= obj.Number55.value.replace(/,/,".");
var num56= obj.Number56.value.replace(/,/,".");
var num57= obj.Number57.value.replace(/,/,".");
var num58= obj.Number58.value.replace(/,/,".");
var select = document.querySelector('#qr2');
var val = [{a : num55},{a :num56 },{a : num57},{a : num58}][select.value];
var qr2 = val.a;
obj.res1578.value= qr2;
var msel = document.getElementById("r1");
var q = msel.value;
obj.res2.value= q;
}
</script>
<form >
<select id="qr2" >
<option value="0"selected>10</option>
<option value="1">12</option>
<option value="2">14</option>
<option value="3">16</option>
</select>
<input type="text" style="width:55px" id="res1578"><br><br>
10 <input type="text" value="1.9" style="width:26px" name="Number55"> 12 <input type="text" value="1.4" style="width:26px" name="Number56">
14 <input type="text" value="1.56" style="width:26px" name="Number57"> 16 <input type="text" value="0.98" style="width:26px" name="Number58">
<br><br>
<select id="r1">
<option value="10">0.1</option>
<option value="2">0.5</option>
<option value="1">1</option>
</select>
<input type="text" style="width:55px" id="res2">
<br><br>
<input type="button" name="WB" value="Ok" onclick="calculat(form)">
</form>
</html>