Calculating and Pasting the Result of a Formula in a Text Document
From LibreOffice Help
If your text already contains a formula, for example "12+24*2", LibreOffice can calculate, and then paste the result of the formula in your document, without using the Formula Bar.
- Select the formula in the text. The formula can only contain numbers and operators and cannot contain spaces.
- Choose Tools - Calculate, or press Ctrl+Plus Sign (+).
- Place the cursor where you want to insert the result of the formula, and then choose Edit - Paste, or press Ctrl+V.
The selected formula is replaced by the result.