Form Validation - Qty In Multiples

Can someone help me write a form validation for a shopping cart that will check if the quantity a user enters is in the correct multiple.

For instance Item A can only be ordered in multiples of 10 - so 10, 20 , 30, etc. and item B can only be ordered in multiples of 5 so 5, 10, 15, etc.

I currently am using a minimum quntity validation and would like to just swap it with the multiples validation - here is the minimum qty validation I am using:

<SCRIPT TYPE="text/javascript">
function valuevalidation(entered, min, max, alertbox, datatype)
{
with (entered)
{
checkvalue=parseFloat(value);
if (datatype)
{smalldatatype=datatype.toLowerCase();
if (smalldatatype.charAt(0)=="i") {checkvalue=parseInt(value)};
}
if ((parseFloat(min)==min && checkvalue<min) || (parseFloat(max)==max && checkvalue>max) || value!=checkvalue )
{if (alertbox!="") {alert(alertbox);} return false;}
else {return true;}
}
}
</script>

And if someone really wants to be my hero I would also like it to alert the user if they use a non-numerical character such as a comma or letter or leave it blank by accident.