How would you solve the problem with paper and pencil?
Once you know that, you can do the same calculation in Excel.

Enter the known values (e.g. £240.43, 20) in cells in an Excel worksheet.
In another cell, enter a formula, starting with an equal sign, and
referring to the cells where you entered the known values.
For example, to add the values in cells A1 and B1:

    =A1+B1