Combinatorial Mathematics : Distinct Elements and Combinations - Four numbers are selected from the set: {-5,-4,-3,-2,-1,1,2,3,4} . In how many ways can the selections be made so that the product ...

5. Four numbers are selected from the set: {-5,-4,-3,-2,-1,1,2,3,4} . In how many ways can the selections be made so that the product of the numbers is positive and:

a) The numbers are distinct.
b) Each number may be selected as many as four times.
c) Each number may be selected at most three times.