A person claims the extrasensory ability of looking at a photograph and telling whether the subject of the photograph is still living or has died. In an experiment to test her claimed ability, she is shown 10 photographs of people unknown to her. (To improve the experiment, the subjects should be of the same age and the photographs taken at the same time; a high-school yearbook would meet both conditions.) She is asked to point out the 5 subjects who are now dead.
a. How many ways can she select 5 of the 10 photographs?
b. How many ways can she select the photographs of the 5 dead subjects?
c. What is the probability of selecting the correct 5 photographs by guessing rather than by extrasensory ability?
d. Why should this be a double-blind experiment?