To determine the relative frequency of the percent of 7th graders who prefer mountains, we need to use the following formula:
Relative frequency = (Number of 7th graders who prefer mountains / Total number of students) * 100
So, relative frequency = (40 / 310) * 100
relative frequency = 0.129 * 100
relative frequency = 12.9%
Therefore, the relative frequency of the percent of 7th graders who prefer mountains is approximately 12.9%.