I will explain you how to answer the first question. The problem says about "every 7 students". According to the condition, you can divide the entire body of 35 students in 5 group, each comprising of 7 students, in a way that 6 out of 7 students in each group turn in their field trip form. It means that the number of students turned in their trip form is 6 multiplied by the number of groups, which is 5. So, the ANSWER to this question is 6*5 = 30 students.