Question 224734
First, you want to identify your fixed and variable costs.  Your fixed cost is the purchase of the phone.  This is a $20 one-time purchase.  No matter how many months you need to use your phone, you will only pay this $20 once.  On the other hand, you will pay $30 each month you are using the phone so this is your variable cost.  Using this information, set up the equation:

C(t) = 30t + 20