Ineed understanding this. i’m kinda slow

Write a story that matches with the expression 42x-5 pls hurry

Simplify. (-2)^-3 a. -8 b. -1/8 c. 1/8 d. 8

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?