Investigators have determined that 15% of a population (say, in a hospital) has chronic disease. Chronic disease patients are encouraged to get an annual flu shot, so the vaccination rates among people with a chronic disease is moderately higher compared to patients without chronic disease. Specifically, among people with chronic disease, about 40% get an annual flu shot. Among people who do NOT have chronic disease, only about 21% of people get an annual flu shot.

This information can be summarized in symbols: P(CD)= 0.15, P(FS|CD)=0.40, P(FS|no CD)= 0.21

Find the probability that a person chosen at random does NOT get an annual flu shot, given that they do NOT have a chronic disease. In other words, find P(no FS| no CD). You may find it helpful to draw a tree diagram.