Use → → to make the table look like a spreadsheet.

Click in cell E2 and use → → (Ctrl+Shift+I) to insert a new formula at caret position.
The Formula Editor is displayed:

After sign '=', type A2*D2 and then click (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:
The node path bar displays: /html/body/table/tbody/tr/td/#processing-instruction(xxe-formula), which means that a formula is processing instruction xxe-formula (more on this later).
A red line is drawn around the F icon, which means that you have explicitly selected this processing instruction.
The status bar displays: =(A2*D2). The small F icon is in fact a special purpose button (more on this later).
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:

Click in cell E5 and insert formula: =sum(E2:E4)
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.
Click in cell E7 and insert formula: =E5+E6
