Sometimes you don't have access to a graphical calculator, or simply you are just too lazy to click the calculator icon. ;-)
Just use the expr command from the command line.
Here some examples, valid under the bash shell, using another shell may require modifications:
Use either simple quotes ' or escape \ for the the *, (, and ) characters, which normally have another meaning in the shell.
All the operators and numbers must be separated by space.
Read more: man expr