4. (5+5 pts). Suppose that 15% of the families in a certain community have no children, 20% have 1, 35% have 2 and 30% have 3. Suppose in each family each child is equally likely to be boy or a girl. If a family is randomly chosen, then let X be the number of girls and Y be the number of boys in this family. a) What is the joint probability distribution of X and Y? b) Are these in dependent