After you answer the above question, here comes a related

来源: 于 2009-12-06 19:07:30 [博客] [旧帖] [给我悄悄话] 本文已被阅读：次

one:
each of the 100 people visits this forum with prob. 0.5. What is the standard deviation for the total number of times these people visit the forum?
ha