The average of 10 observations is 40. It was realized later that an observation was misread as 34 in place of 43. Find the correct average.

Correct option is D

Given:
The average of 10 observations is 40.
One of the observations was misread as 34 in place of 43.
Formula Used:
​$$\text{Average} = \frac{\text{Sum of all observations}}{\text{Number of observations}}$$​​
Solution:
Sum of observations = Average × Number of observations = 40 × 10 = 400
Correct sum = 400 + (43 - 34) = 400 + 9 = 409
Correct average = $$\frac{409} {10} = 40.9$$ 
Alternate Solution:
Correct Average = $$40 + \frac{-34 + 43}{10} = 40 + \frac{9}{10} = 40.9$$​​