Identify dependent & independent variables

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You are buying a gym membership. The membership costs $30\$30 per month. In the graph at the bottom of the page, dd is the total number of dollars that you pay for your gym membership, and tt is the time (in months) that you keep the membership.
Write an equation for the amount of money, dd, that you pay for your gym membership if you keep the membership for tt months.
Then, complete the table below for the missing value of dd.
ttdd
113030
226060
339090
............
2424