Here is what i THINK it wants: I think that each x represents the number of each material purchased and what you are looking for is the frequency at which each item was bought each month. Again, this is my guess but this question is very strange.
I believe that the number of students are the numbers on the left and the x's are amount ($6) spent that month so I multiplied the amount of students with the number of X's or 6 and I found the there are 70 students in this and all together they spent $27780 in 4 months.
