Question

The mean of 200 observations is 20. It is found that the value of 180 is wrongly copied as 280. Find the actual mean.

Answer 1

Since mean of 200 observations = 20
⇒ Sum of 200 observations 
= 200 x 20
= 4000
⇒ Incorrect sum = 4000
∴ Correct sum
= Incorrect sum - Incorrect value + Correct value
= 4000 - 280 + 180
= 3900
∴ Correct mean

= `"Correct sum"/"Total number of observations"`

= `(3900)/(200)`
= 19.5.