A study was designed to investigate the effects of two variables−(1) a student's level of mathematical anxiety and (2) teaching method−on a student's achievement in a mathematics course. Students who had a low level of mathematical anxiety were taught using the traditional expository method. These students obtained a mean score of 340 with a standard deviation of 30 on a standardized test. Assuming no information concerning the shape of the distribution is known, what percentage of the students scored between 280 and 400?