18. Suppose that the remission time, in weeks, of leukemia patients that have undergone a certain type of chemotherapy treatment is an exponential random variable having an unknown mean θ. A group of 20 such patients is being monitored and, at present, their remission times are (in weeks) 1.2, 1.8∗, 2.2, 4.1, 5.6, 8.4, 11.8∗, 13.4∗, 16.2, 21.7, 29∗, 41, 42∗, 42.4∗, 49.3, 60.5, 61∗, 94, 98, 99.2∗ where an asterisk next to the data means that the patient’s remission is still continuing, whereas a data point without an asterisk means that the remission ended at that time. What is the maximum likelihood estimate of θ?