868 is about 870, and 28 is about 30, so your expression would be 870 divided by 30 which would be 29 and the actual answer is 31, so your estimate would be close to your actual answer.
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