The question is asking for mass/unit of volume. Therefore the density of the material would be 6400kg/0.8m^3. You then want to change the bottom into a 1 by multiplying the top and bottom both by 10/8. You now have 8000kg/m^3, which is the solution.
I'm not completely sure about this answer but here is what I got: 100•15=1500 for one day of the food booth. 50•22=1100 one day for the game booth. 1500+(x9) and 1100+(x8). (1500+x)+(1100+x)=x
