interest rate calculator

I am trying to create a calculator that allows the user to enter a balance amount, the interest rate and the payment. I want the calculator to figure and write the amount that went to interest and the amount that went to the balance. My biggest problem is that I need the two calculations to stop after two decimals. I can't figure it out.

The calculator can be found at costofcredit.com/interest_paid.html

Thanks for any help.

For the decimal you can use this:

tmp = 345.2342355;
tmp2 = Math.floor(tmp*100)/100;

results:

tmp2 = 345.23

Thanks for replying. Can you help me with the code I wrote. My problem is that I can't figure out how to write the code with a undefined value. Here's my code:


<script language="Javascript">

<!-- begin

function checkNumber(input, min, max, msg)
{msg = msg + "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 = parseFloat(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.balance.value==null || form.balance.value.length==0) ||
(form.interest.value==null || form.interest.value.length==0) ||
(form.payment.value==null || form.payment.value.length==0)
)
{return;}

if ( !checkNumber(form.balance, 1, 100000, "Balance") ||
!checkNumber(form.interest, .001, 99, "Interest") ||
!checkNumber(form.payment, 10, 100000, "Payment")
)
{form.interestpaid.value = "Invalid"; return;}

var i= form.interest.value;
if(i > 1.0)
{i = i / 100.0; form.interest.value = i;}


i /=12;
money = (form.balance.value * i * 1.00);
form.interestpaid.value = money;
applied = (form.payment.value - money);
form.appliedtobalance.value = applied;
}

function clearForm(form)
{form.balance.value = "";
form.interest.value = "";
form.payment.value = "";
}

//done hiding from old browsers-->
</script>