If C of t is equal to 180 plus 10 t represents ISP A and C of t is equal to 25 t represents ISP B, how long would the service contracts need to be for the total costs to be the same?

Set the two cost equations equal to find when the total costs match: 180 + 10t = 25t. Subtract 10t from both sides to get 180 = 15t, so t = 12. The contracts have the same total cost after 12 months, which is choice C.

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