Media service charges $20 for a phone and $30/month for its economy plan. The equation c=30t +20 describes the total cost,c, of operating Media Services phone for t months.

The answer would be B (2 1/3) if the phone company billed for partial months or refunded the unused part of a month. But that is not how most of them work. If payment must be in whole months, the answer would be A.

To find the number of months of service a customer can receive with a given amount of money, you need to solve the equation c = 30t + 20 for t (number of months), where c is the total cost.

Given that a customer has $90 available, you can substitute c = 90 into the equation and solve for t:

90 = 30t + 20

Subtracting 20 from both sides:

90 - 20 = 30t

70 = 30t

Dividing both sides by 30:

70/30 = t

t ≈ 2.33

So, the customer can receive approximately 2.33 months of service. Rounding to the nearest option, the answer is B. 2 1/3.