75. Hope Middle School needs to select exchange students from Grade 7 to Grade 12, but due to the limited number of slots, the following rules are set:
(1) At most 3 students from Grade 7;
(2) At most 4 students from Grade 8;
(3) At most 5 students from Grade 9;
(4) At most 8 students from Grade 10;
(5) At most 10 students from Grade 11;
(6) At most 7 students from Grade 12.
If no matter how the selection is made, at least one of the above rules must be followed, then the maximum number of students that can be selected is $\qquad$.
answer: 42