First you need to know the total number of students. This is 40 students times 6 classes
40 x 6 = 240 students
if each student needs 8ml of solution
240 students x 8ml = 1920ml
Since you can only order by the L you need to convert 1920ml to L
there are 1000ml in each L
1920ml 1L
------------ x ------------
1 1000ml
the two ml units cancel and we get
1920L
--------- = 1.92L
1000
Since the solution can only be ordered by the liter we have to round up to
2L