<!-- hide this script tag's contents from old browsers

function bank(num)

{

   var1 = (num * 100) % 1;
   var2 = (num * 100) - var1;
   if (var1 > 0)
   {var2 += 1;}
   num = var2 / 100;
   return num;

}

function checkNumber(input, min, max, msg)

{

    msg = msg + " field has invalid data: " + input.value;
    var str = input.value;
    for (var i = 0; i < str.length; i++) {

        var ch = str.substring(i, i + 1)
        if ((ch < "0" || "9" < ch) && ch != '.') {

            alert(msg);
            return false;

        }

    }

    var num = 0 + str
    if (num < min || max < num) {

        alert(msg + " not in range [" + min + ".." + max + "]");
        return false;

    }

    input.value = str;
    return true;

}

function computeField(input)

{

    if (input.value != null && input.value.length != 0)

        input.value = "" + eval(input.value);

    computeForm(input.form);

}

function computeForm(form)

{

    if ((form.annum.value == null || form.annum.value.length == 0) ||

        (form.interest.value == null || form.interest.value.length == 0) ||
        (form.purchase.value == null || form.purchase.value.length == 0)) {
        return;

    }

    if ((form.down.value == null) || form.down.value.length == 0)

         {form.down.value = 0;}

    if (!checkNumber(form.annum, 1, 40, "# of years") ||

       !checkNumber(form.interest, .001, 99, "Interest") ||

        !checkNumber(form.purchase, 25, 100000000, "Purchase")) {
        form.payment.value = "Invalid";
        return;

    }

    var i = form.interest.value;
    if (i < 1.0)

      {

        i = i * 100;
        form.interest.value = i;

      }

    i = form.interest.value / 100;
    pow = 1 + (i/2);
    i = Math.pow(pow, 1/6) - 1;
    bot1 = (1 / (1 + i))
    bot2 = Math.pow(bot1, (form.annum.value * 12))
    form.payment.value = ((form.purchase.value - form.down.value)* i) / (1 - bot2);
    form.payment.value = bank(form.payment.value);

}

function clearForm(form)

{

    form.annum.value = "";
    form.interest.value = "";
    form.purchase.value = "";
    form.down.value = "";
    form.payment.value = "";

}

<!--done hiding from old browsers -->