Answer:
c = 55 . . . . (no units for cost are given in your problem statement)
Explanation:
"t is the number of tickets" and "Marci bought 20 tickets", so we conclude t=20. Putting that value into the equation and doing the arithmetic, we find ...
c = 2.75·20 = 55
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We don't know if this is pennies or pounds, dollars or rupees or pesos or euros. No monetary units are mentioned in the problem statement given here.