Intensive care units (ICUs) generally treat the sickest patients in a hospital. ICUs are often the most expensive department in a hospital because of the specialized equipment and extensive training required to be an ICU doctor or nurse. Therefore, it is important to use ICUs as efficiently as possible in a hospital. According to a 2017 large-scale study of elderly ICU patients, the average length of stay in the ICU is 3.4 days (Critical Care Medicine journal article). Assume that this length of stay in the ICU has an exponential distribution. 

a. What is the probability that the length of stay in the ICU is one day or less? 

b. What is the probability that the length of stay in the ICU is between two and three days? 

c. What is the probability that the length of stay in the ICU is more than five days?