Let's call S the number of all the staplers that have been used by the students, R the number of all rulers that have been used by the students, G the number of glue bottles that have been used by the students and X the number of all the students.

Since every 2 students have to share a stapler, then:

1 stapler has to be shared by 2 students, so:

S staplers has to be shared by X students, so:

2S = X

You do the same with R and X and G and X.

You also know that S+R+G=65 so then you can solve it in a straightforward way.

Why exactly?I have no idea how to do this. If i add 2+3+4= 9. So number of students has to be a multiple of 9.