If we know the cost of a ticket is $7, we can then write an equation in slope-intercept form for the amount it will cost to buy any amount of tickets.
If we denote x as the amount of tickets purchased, and c as the total cost, we can write the equation:
Since we can vary the amount of tickets purchased independently of the total amount, x (the amount of tickets purchased) will be the independent variable.
However, since y (the total cost) is directly dependent on the amount of tickets that are purchased, c is the dependent variable.