Final answer:
The equation to represent the balance on the prepaid Starbucks card after each week of gas expenses is y = 200 - 40x.
Step-by-step explanation:
To represent the balance on the prepaid Starbucks card, we need to subtract the weekly gas expenses from the initial amount of $200. Each week, Sarah's daughter spends $40 on gas, so the equation to represent the balance on the card after each week, x, is y = 200 - 40x.