Continental Mobile offers an optional call package whereby each month the subscriber gets the first 50 minutes free, the next 100 minutes at $0.25/min, and any additional time at the normal rate of $0.50/min. What is the opportunity cost of making an additional 20 minutes of calls for a subscriber who currently makes Instructions: Round your answers to 2 decimal places. . a. 40 minutes of calls each month? . $ b. 140 minutes of calls each month? $