# NCERT Class 11 Mathematics Solutions: Chapter 15 –Statics Miscellaneous Exercise Part 4

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1. The mean and standard deviation of observations are found to be and , respectively. On rechecking, it was found that an observation was incorrect. Calculate the correct mean and standard deviation in each of the following cases:

(i) If wrong item is omitted.

(ii) If it is replaced by .

Answer: (i)

Number of observations

Incorrect mean

Incorrect standard deviation

That is, incorrect sum of observations

Correct sum of observations

Correct Mean

Standard deviation

Answer: (ii)

When is replaced by ,

Incorrect sum of observations

Correct sum of observations

Correct Mean

Standard deviation