A single checkbox field stores in the database as 1 (checked) or 0 (unchecked). Multicheckbox will store whatever the option Value is set to. All substitutions must be treated as strings so surround them with single or double quotes as needed. If you expect them to be an integer you can typecast them to (int) as done in the example code I've already provided. So for example to multiply a field value you'd use the below.
Code:
return (int) '[cb_field1]' * 2;
See the below for further operator usage information.
php.net/manual/en/language.operators.arithmetic.php
www.w3schools.com/php/php_operators.asp
Why do you need to store the results of your math? If it's only for display purposes simply use a Code field. If you need it stored then use CB Auto Actions acting on the after profile update trigger with a Field action then enable format functions and use the math format function.