131. De ne a function p(x; l, m) by

a. Show that p(x; l, m) satis es the two conditions necessary for specifying a pmf. [Note: If a rm employs two typists, one of whom makes typographical errors at the rate of l per page and the other at rate m per page and they each do half the rm s typing, then p(x; l, m) is the pmf of X  the number of errors on a randomly chosen page.]

b. If the rst typist (rate l) types 60% of all pages, what is the pmf of X of part (a)?

c. What is E(X) for p(x; l, m) given by the displayed expression?

d. What is s2 for p(x; l, m) given by that expression?