5. (total 16 points)
brand w: 500 napkins for $2.87
brand x: 200 napkins for $1.27
brand y: 100 napkins for $0.62
brand z: 250 napkins for $1.37
a. which brand is the cheapest? what is the cost of one napkin of this brand? (total 8 points; 4 points for the answer, 2 points for the units labeled and 2 points for work shown or explanation)
b. which brand is the most expensive? what is the cost of one napkin of this brand? (total 8 points; 4 points for the answer, 2 points for the units labeled and 2 points for work shown or explanation)