A friend of yours is considering two cell phone service providers. Provider A charges $120 per month for the service regardless of the number of phone calls made. Provider B does not have a fixed service fee but instead charges $1 per minute for calls. Your friend'smonthly demand for minutes of calling is given by the equationQD=150-50P, wherePis the price of a minute. With each provider, what is the cost to your friend of an extra minute on the phone?