6.45 30 students from 5 grades participate in answering 40 questions. Each student answers at least 1 question, every two students from the same grade answer the same number of questions, and every two students from different grades answer a different number of questions. How many people answered only 1 question?
[Solution] 30 students each answered 1 question, totaling 30 questions. 5 grades of students, with different grades answering different numbers of questions, answered at least 5, 4, 3, 2, 1 questions respectively. But when the people who solved 5, 4, 3, 2 questions each have 1 person, a total of 14 questions have been solved. Therefore, 26 people only solved 1 problem.