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The mean and standard deviation of 100 observations are 40 and 5.1, respectively. By mistakeone observation is taken as 50 instead of 40. If the correct mean and the correct standarddeviation are $\mu$ and $\sigma$ respectively, then $10(\mu +\sigma)$ is equal to

The correct answer is
449

Calculating Correct Mean ($\mu$)

We are given:

  • Number of observations, $n = 100$
  • Incorrect Mean, $\bar{x}_{inc} = 40$
  • Incorrect standard deviation, $s_{inc} = 5.1$
  • Incorrect observation recorded = 50
  • Correct observation = 40

First, calculate the sum of the incorrect observations:

$ \text{Sum}_{inc} = n \times \bar{x}_{inc} = 100 \times 40 = 4000 $

Now, adjust the sum to find the correct sum:

$ \text{Sum}_{corr} = \text{Sum}_{inc} - (\text{Incorrect Observation}) + (\text{Correct Observation}) $ $ \text{Sum}_{corr} = 4000 - 50 + 40 = 3990 $

Calculate the correct mean ($\mu$):

$ \mu = \frac{\text{Sum}_{corr}}{n} = \frac{3990}{100} = 39.9 $

Calculating Correct Standard Deviation ($\sigma$)

We use the formula for variance: $Var = \frac{\sum x^2}{n} - (\text{Mean})^2$.

First, find the incorrect sum of squares ($\sum x^2_{inc}$):

$ \text{Variance}_{inc} = (s_{inc})^2 = (5.1)^2 = 26.01 $ $ \sum x^2_{inc} = n \times (\text{Variance}_{inc} + (\bar{x}_{inc})^2) $ $ \sum x^2_{inc} = 100 \times (26.01 + (40)^2) = 100 \times (26.01 + 1600) = 100 \times 1626.01 = 162601 $

Adjust the sum of squares to find the correct sum of squares:

$ \sum x^2_{corr} = \sum x^2_{inc} - (\text{Incorrect Observation})^2 + (\text{Correct Observation})^2 $ $ \sum x^2_{corr} = 162601 - (50)^2 + (40)^2 = 162601 - 2500 + 1600 = 161701 $

Now, calculate the correct variance:

$ \text{Variance}_{corr} = \frac{\sum x^2_{corr}}{n} - \mu^2 $ $ \text{Variance}_{corr} = \frac{161701}{100} - (39.9)^2 = 1617.01 - 1592.01 = 25 $

Calculate the correct standard deviation ($\sigma$):

$ \sigma = \sqrt{\text{Variance}_{corr}} = \sqrt{25} = 5 $

Final Calculation: $10(\mu + \sigma)$

Substitute the values of $\mu$ and $\sigma$:

$ 10(\mu + \sigma) = 10(39.9 + 5) = 10(44.9) = 449 $

Thus, the value of $10(\mu + \sigma)$ is 449.

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