Multiply TextBox Values in JavaScript to calculate Interest rates

Here I have created sample that calculate Total amount From Three Textboxes Value.

HTML

<div>
    <script type="text/javascript">
        function Calculate() {
            var quantity = document.getElementById('<%= txtQuantity.ClientID%>').value;
            var rate = document.getElementById('<%= txtRate.ClientID%>').value;
            var inr = document.getElementById('<%= txtINRRate.ClientID%>').value;
            if (quantity != '' && quantity != "" && inr != "") {
                document.getElementById('<%= txtTotal.ClientID%>').value = eval((parseInt(quantity) * parseInt(rate) * parseInt(inr)));
            }
            else {
                document.getElementById('<%= txtTotal.ClientID%>').value = 0
            }
            return false;
        }
    </script>
    <table border="0" cellpadding="0" cellspacing="0">
        <tr>
            <td>
                Quantity :
            </td>
            <td>
                <asp:TextBox ID="txtQuantity" runat="server" />
            </td>
        </tr>
        <tr>
            <td>
                Rate :
            </td>
            <td>
                <asp:TextBox ID="txtRate" runat="server" />
            </td>
        </tr>
        <tr>
            <td>
                INR Rate :
            </td>
            <td>
                <asp:TextBox ID="txtINRRate" runat="server" />
            </td>
        </tr>
        <tr>
            <td colspan="2">
                <asp:Button ID="btnCalculate" runat="server" Text="Calculate" OnClientClick="return Calculate()" />
            </td>
        </tr>
        <tr>
            <td>
                Total :
            </td>
            <td>
                <asp:TextBox ID="txtTotal" runat="server" />
            </td>
        </tr>
    </table>
</div>

Screenshot