There is one ball in every cereal package. The colors of these balls are yellow, blue, green, red and black. 40% of the balls are yellow, 25% blue, 15% green, 15% red and 5% black. What is the probability that a) a person doesn't get yellow ball even if he opens 4 packages b) he needs to open exactly 5 packages to get a yellow ball c) after opening 6 packages, he has two blue, three yellow and one green ball d) after opening 3 packages, he has at least 1 yellow and 1 blue ball?