Final answer:
The range of Jenny's debit card balance after setting a weekly budget of $20 for lunches would be $280 to $300, since that reflects the lowest and highest amounts possible after her weekly spending.
Step-by-step explanation:
Considering Jenny's financial situation, we should look at the range of money she can have on her debit card after allocating her budget of $20 cash each week for school lunches. Initially, Jenny's parents gave her a debit card with $300. If she uses all her cash budget without spending from her debit card, the amount on the card stays the same, which is $300. However, if she needs to use her debit card because she used up her cash, the lowest amount she could have left on the card after spending her weekly budget would be $300 - $20, which equals $280. Therefore, the range of the money on her debit card after setting her weekly budget would be from $280 to $300.