totalOrder += (item.Quantity * item.SellingPrice);

total = (totalOrder * 8%);

var tax = .08;
var tax = 8%

 ((totalOrder * tax) / 100);

<tbody>
                            @foreach (var item in Model.InvoiceDetail)
                            {
                                <tr>
                                    <td>@Html.DisplayFor(modelItem => item.Quantity)</td>
                                    <td>@Html.DisplayFor(modelItem => item.Material)</td>
                                    <td>@Html.DisplayFor(modelItem => item.Description)</td>
                                    <td>$@Html.DisplayFor(modelItem => item.SellingPrice)</td>

                                    <td>@string.Format("{0:C2}", (item.Quantity * item.SellingPrice))</td>
                                </tr>
                                totalOrder += (item.Quantity * item.SellingPrice);

                                total = (totalOrder * 8%);
                            }
                        </tbody>
                    </table>
                    <h5 style="margin-left: 80%;"><b>Sub Total :</b> @string.Format("{0:C2}", totalOrder)</h5>
                    <h5 style="margin-left: 80%;"><b>Tax :</b> 8%</h5>
                    <h5 style="margin-left: 80%;"><b>Total Due :</b> @string.Format("{0:C2}", total)</h5>

decimal totalOrder = 0;
decimal total = 0;

decimal totalOrder = 100.1m;
decimal tax = .08m;
decimal total = totalOrder * (1.0m + tax);

<tbody>
                            @foreach (var item in Model.InvoiceDetail)
                            {
                                <tr>
                                    <td>@Html.DisplayFor(modelItem => item.Quantity)</td>
                                    <td>@Html.DisplayFor(modelItem => item.Material)</td>
                                    <td>@Html.DisplayFor(modelItem => item.Description)</td>
                                    <td>$@Html.DisplayFor(modelItem => item.SellingPrice)</td>

                                    <td>@string.Format("{0:C2}", (item.Quantity * item.SellingPrice))</td>
                                </tr>
                                //totalOrder += (item.Quantity * item.SellingPrice);


                            }
                        </tbody>
                    </table>
                    @{totalOrder += (Model.TotalCount * Model.PricePer);}
                    <h5 style="margin-left: 80%;"><b>Sub Total :</b> @string.Format("{0:C2}", totalOrder)</h5>
                    <h5 style="margin-left: 80%;"><b>Tax :</b> 8%</h5>
                    <h5 style="margin-left: 80%;"><b>Total Due :</b> @string.Format("{0:C2}", total)</h5>

decimal totalOrder = 100.01m;
decimal tax = .08m;
decimal total = totalOrder * (1.0m + tax);

decimal totalOrder = 0m;
decimal tax = .08m;
@{decimal totalTax = totalOrder * (tax);
  decimal total = totalOrder + totalTax;}

<tbody>
                            @foreach (var item in Model.InvoiceDetail)
                            {
                                <tr>
                                    <td>@Html.DisplayFor(modelItem => item.Quantity)</td>
                                    <td>@Html.DisplayFor(modelItem => item.Material)</td>
                                    <td>@Html.DisplayFor(modelItem => item.Description)</td>
                                    <td>$@Html.DisplayFor(modelItem => item.SellingPrice)</td>

                                    <td>@string.Format("{0:C2}", (item.Quantity * item.SellingPrice))</td>
                                </tr>
                                totalOrder += (item.Quantity * item.SellingPrice);


                            }
                        </tbody>
                    </table>
                    @{decimal totalTax = totalOrder * (tax);
                        decimal total = totalOrder + totalTax;}
                    <h5 style="margin-left: 80%;"><b>Sub Total :</b> @string.Format("{0:C2}", totalOrder)</h5>
                    <h5 style="margin-left: 80%;"><b>Tax :</b> 8%</h5>
                    <h5 style="margin-left: 80%;"><b>Total Due :</b> @string.Format("{0:C2}", total)</h5>