求解一道GRE数学题

Jennifer is playing a game in which she will choose a ball from a bag containing 10 balls, each of which is labeled with a different integer from 1 to 10. If she chooses a ball labeled 1, 2 or 3, she will lose 10 points. If she chooses a ball labeled 4 or 5, she will gain 5 points. If she chooses a ball labeled 6, 7, 8, 9, or 10, she will gain 20 points. What is the expected value of the number of points Jennifer will gain by choosing a ball at random from the bag?
答案是9. 但是我怎么算都是8. 哪一位能够分析一下， 到底是8， 还是9？