An internet service provider charges $150 per month for Cable connection. Variable costs are $50 per account per month. Suppose that the provider has a marketing spend of $50 per account per year, this internet provider has 20,000 customers but is losing 1,000 customers every year to competitors. The monthly discount rate is 5%. Customers are billed at the end of each month.

a) Calculate margin per customer

b) Calculate the retention rate

c) What is the customer lifetime value? Based on monthly periods.