在有奖摸彩中，一期(发行10000张 * * 为一期)有200个奖品是5元的，20个奖品是25元的，5个奖品是100元的．在不考虑获利的前提下，一张 * * 的合理价格是多少元？
0．2元．
解：设一张 * * 中奖额为随机变量ξ，显然ξ所有可能取的值为0，5，25，100
依题意，可得ξ的分布列为
| ξ | 0 | 5 | 25 | 100 |
| P |  |  |  |  |
所以一张 * * 的合理价格是0．2元．
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My name is Rich . I'm fourteen years old . I 'm __1__ in a middle school . My school is nice and clean. There are 30 students in my class . It's __2__ today . My classmates and I are __3__ a good time on the beach .It's a nice place .The water is blue and clear . Many birds are __4__ over the sea . Many people are __5__ . Some boys are __6__ football . Some people are __7__ on the beach . A girl is __8__ pictures on a chair .Another girl is __9__ photos . I'm __10__ to you ! What are you doing ? I hope you can write to me soon . Best wishes! Rich |