See the sampling from a population link under the 'normal distribution' section
http://www.hippocampus.org/course_locator?course=Statistics%20for%20Social%20Sciences&lesson=6&topic=1&width=800&height=684&topicTitle=Sampling%20from%20a%20Population&skinPath=http://www.hippocampus.org/hippocampus.skins/default