function computeCost(form) {    
    var num_cars =    Math.round((0.5 * form.Guests.value)); 
    form.t_cost.value = "0";
    
    //if (IsNumeric(form.Guests.value) ) {
        if (form.parking.value == "1" ) {
            form.t_cost.value = (Math.ceil(num_cars / 20)) + 1;
        }
        if (form.parking.value == "2" ) {
            form.t_cost.value = (Math.ceil(num_cars / 15)) + 1;
        }
        if (form.parking.value == "3" ) {
            form.t_cost.value = (Math.ceil(num_cars / 10)) + 1;
        }
        form.t_cost.value = 75 * form.t_cost.value;
    //} 
   return;
}

function clearCost(form) {    
    form.Guests.value = "";    
    form.parking.value = "0";   
    form.t_cost.value = "";   

    return;
}


function IsNumeric(sText) {
    var ValidChars = "0123456789.";
    var Char;
    alert(sText);
    
    for (i = 0; i < sText.length && IsNumber == true; i++){
        Char = sText.charAt(i);
        if (ValidChars.indexOf(Char) == -1){
            return false;
        }
    }
    return true;
}