The following is an example of a calculation in LibreOffice Calc.
Click in a cell, and type a number
The cursor moves down to the next cell.
Enter another number.
Press the Tab key.
The cursor moves to the right into the next cell.
Type in a formula, for example, =A3 * A4 / 100.
The result of the formula appears in the cell. If you want, you can edit the formula in the input line of the Formula bar.
When you edit a formula, the new result is calculated automatically.