We have to assume that m is supposed to stand for the money (in dollars) that results from the sale of t tickets. Then the sale of t=1 ticket will result in m=15 dollars. Only one of the offered formulas will give this result:
m = 15t
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Comment on the problem
Variables need to be defined. We'd prefer to see some discussion of how concert expenses are covered, so we can be assured that ticket sales directly relate to money raised.
If the number of tickets is t and the total amount of money collected is m then a product of the price per ticket with the cost of the ticket gives the amount of money collected from the sale of the tickets.
