完成活动A所需的时间，悲观（P）的估计需36天，最可能（ML）的估计需21天，乐观（O）的估计需6天。活动A在16天至26天内完成的概率是（）
A．55.70%
B．68.26%
C．95.43%
D．99.73%
参考答案：B
解析：
在PERT网络计划中，某些活动或全部工序的持续时间事先不能准确肯定，适用于不可预知因素较多的、过去未曾做过的新项目或复杂的项目，或研制新产品的工作中。
使用PERT计划评审技术估算某活动i,j的历时Di,j，其公式为：
[*]
其中：
mij为完成活动i,j所需的最可能时间；
aij为完成活动i,j所需的乐观估计时间：
bij为完成活动i,j所需的悲观估计时间。
其估算的方差为：
[*]
根据上述公式，完成活动A需要的历时为：
(4*21+6+36)/6=126/6=21天
其估算的方差为：
(36-6)/6=5天
因PERT方法估算出的活动历时概率分布符合标准的正态分布，故活动A在一个方差(21-5，21+5)内完成的概率为68.26%。
Dash of Hope “Dash” is a symbol which represents every day we’ve spent alive on earth. Therefore, how you spend your “dash” is important. Recently I 36 about a little girl named Hope. After learning more about her life, I couldn’t help but feel it was not by 37 , nor happenstance（意外事件）, that she had been named “Hope”. The strong feeling of sympathy and generosity 38 in her young heart made a lasting impression on me and countless others. 39 I never had the opportunity to meet her, I wish I had. It seems as though she was wise beyond her tender years and very, very special. Hope was a twelve-year-old girl who was 40 a “wish” in early December 2010 by the “Make-A-Wish” Foundation after being 41 that she had a rare type of bone cancer. However, when she found out that more than 150 42 in her area were waiting for their wishes to be achieved , she unselfishly used her wish to 43 that those children have their wishes granted. She also asked that it be done 44 January 16, 2011. Unfortunately, however, the organization informed her that her 45 request could not be granted as the funds were simply 46 . They calculated that they would need to raise more than one 47 dollars in thirty days in order to grant her wish. 48 , but not discouraged, she turned her disappointment into an enthusiasm that inspired caring 49 to take up helping grant the wishes of the other children, and eventually 50 as well. Newspaper columnists and reporters for radio and TV stations 51 the story of this caring young girl who had 52 the hearts of so many and as word spread, the community was challenged. Committees were fanned and schools, corporations and various organizations assisted in 53 money to help make Hope’s dream come true. Her efforts were not in vain as they continue to help others, not only 54 , but spiritually and emotionally as well. At the gathering to celebrate her life, “A Celebration of Hope” on January 16, 2011 , the 55 was made that they had indeed received donations totaling more than one million dollars. Her wish had been granted!
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