Here is a great web page that can be used to create math questions for Moodle. You simply give it your limitations for the numbers in the question and it will generate an XML file that can be imported into Moodle. It can generate addition, subtraction, multiplication, division, powers or exponents, or square root problems. You can provide further limitations to get answers that are only integers, or only positive numbers.
http://aschool.us/moodle-scripts/math-questions.php