1. Basics

Select checkbox menu item ToolsSpreadsheetShow Table Labels to make the table look like a spreadsheet.

Click in cell E2 and use ToolsSpreadsheetInsert/Edit Formula to insert a new formula at caret position.

The Formula Editor is displayed:

After sign '=', type A2*D2 and then click OK (shortcut Ctrl+Enter).

You have inserted your first formula in the document. A formula is visually represented by a small F icon. Don't worry if you find it ugly: it will disappear when you'll print the document or when you'll convert it to other formats.

Click on the F icon. Notice that:

Copy the formula (Ctrl+C) to the clipboard and paste it (Ctrl+V) in cells E3 and E4.

Now type in cell A2: 2, tab to cell B2 to type: xe-1u, tab again to cell D2 and type: 200. Do the same in the next two rows: 1,xe-5u,800 and 1,fc-uu,3000. Then click in cell E5 to force an update. The table should now look like this:

1.1. Three more formulas to finish first version of the invoice

  1. Click in cell E5 and insert formula: =sum(E2:E4)

  2. Click in cell E6 and insert formula: =rounddown(E5 * left(D6, len(D6)-1)%, 2)

    left(D6, len(D6)-1) is string "19.6%" without its last character '%'.

    Note that you can use spaces in a formula and that a formula is case-insensitive. For example: SUM(e2:e4) works fine too.

  3. Click in cell E7 and insert formula: =E5+E6