Answers

2014-04-21T17:55:34+05:30
Now the mean of 36 observation is 11.25
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2014-04-21T18:04:52+05:30
Mean of 36 observations = 12
       Sum of 36 observations = 36 * 12 = 432
 Now, actual Sum = Sum of 36 observations + correct observation - incorrect observation
                                          = 432 +47 - 74 = 405
Hence correct mean = Actual sum of 36 observations / 36
                                          405 / 36
                                              = 11.25
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