To find the relative frequency of a certain category (e.g. beach field trip for 7th graders), we need to divide the frequency of that category by the total number of responses.
For the 7th graders and beach field trip:
Relative frequency = 70 / 310 = 0.2258
Rounded to the nearest tenth, the relative frequency of 7th graders choosing the beach field trip is 22.6%.