This program allows you to work out any expression, such as 2 * 3 - (4 - 5 * 3.2) / 8.11. You may include spaces, but remember to put in multiply signs where they are needed (i.e. If you wanted to enter 2(4 - 3), you would have to enter it as 2*(4 - 3).
Use the Source or View Source option on your View menu to see how it works. Then use the Back icon on the toolbar.