4. The "one-child" policy is an official program launched in the late 1970s by the Chinese central government, with the aim of limiting the number of children per family to one child. We consider a family composed of the husband, wife and an adult child and Let X be the number of smokers in this family. The possible values of X are 0,1,2,3. Suppose that the probability mass function of X is given as follows F(x) = c(4-x) For x = 0,1,2,3.