Use formula (pls need it for now urgently ill give brainliest) and explain the process

Adaycare center charges a $75 enrollment fee plus $100 per week. which of the following represents the cost of sending a child to daycare for 14 weeks?

Satchi found a used bookstore that sells pre-owned dvds and cds. dvds cost $9 each, and cds cost $7 each. satchi can spend no more than $45.

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?