A company has started a phone service that uses overseas doctors to provide emergency medical consultations. The responding doctors are based in a country with low wages but with a highly skilled pool of physicians. Responding to each call takes on average 15 minutes. At any given time, there are 4 doctors overseas on duty. Calls arrive every 5 minutes on average (standard deviation is 5 minutes). The company receives $50 from the patient’s insurance company for each consultation. If one of the 4 overseas doctors is available, the firm pays $20 to the doctor and makes $30 in profit. If no doctor is available overseas, the call is rerouted to the U.S. where a local physician answers the question. A local physician is always available to take a call. In this case, the firm pays the $50 to the local physician, so there’s no profit for the company.

What is the percentage of a call being answered by a physician in the US?