Melanie has $1.80 worth of nickels and dimes. She has 12 more nickels than dimes. Graphically solve a system of equations in order to determine x=the number of nickels, and y=the number of dimes that Melanie has.

Last month, a 770 g box of cereal was sold at a grocery store for $3.10. however, this month, the cereal manufacturer launched the same cereal in a 640 g box, which is being sold at $2.50. what is the percent change in the unit price?