Ok so, Shaun should have 4 balls and Anna should have about 12. So, if Shaun takes 4 of Anna's tennis balls then he would have 8, but if Anna takes 4 of Shaun's balls she would get 16. That means the number of tennis balls she has right now would be 12, and if she gets 4 of Shaun's balls she would have 16. Hope This Helps! :)