Answer: 35 seventh graders
Explanation:
The simplified ratio is 4:7 sixth graders to seventh graders
The number of sixth-graders is 5 times the number shown in the ratio of attendees (20/4 = 5). Thus, the number of 7th graders must also be 5 times greater than the simplified ratio.
7*5 = 35, thus there are 35 seventh graders in the club