A study was designed to determine 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 430 with a standard deviation of 30 on a standardized test. Assuming a non-mound-shaped distribution, what percentage of the students scored over 520?
a) at least 89%
b) at most 5.5%
c) at most 11%
d) approximately 2.5%