It is interesting to note that back in 1940 if a person in the United States was selected at random, then his or her age X had a distribution that was almost given by the p. d.f. of the form f(x) = {c 0 < x < 35}
c[1+ 3/140(35-x)], 35 <= x<=81 2/3

Required:
a. Find c so that f(x) is a p. d.f.
b. In 1940, about what percentage of the U. S. population was older than 65?
c. What was the median age then?