For question 3.
Since all rows need to have an equal number of students with all students being from the same grade, find the greatest common multiple for the number of students from each grade.
The greatest common multiple of 50, 25, 75 and 100 is 25.
This means that each row should have 25 students per row, so there will be 2 rows of fifth graders, 1 row of sixth graders, 3 rows of seventh graders and 4 rows of eighth graders.
For question 4.
Since it is saying that the company discounts the same number of instruments per type every month, you just need to find the smallest amount.
That amount is 26 (drums). So the company can discount 26 of each.
Hopefully I helped!