We can write an equation for the balance of the gift card.
We know that if she doesn't buy any ticket (x = 0), the balance will be $100, which is the initial balance.
Each increase in x, is a decrease of 8.75 for the balance. This represents the slope of the equation, as there is not other term.
Then, we can write the equation as:
If she wants to know how many tickets she can buy to have a $15 balance (y = 15), we can use the equation as:
She can buy up to 9 tickets (if she buys 10 tickets, she will have less than $15).
Answer:
The equation is y = 100 - 8.75x.
X = 9 tickets for a balance of $15.