Question

The mean of 4 observations is 20. If one observation is excluded, the mean of the remaining observations becomes 15. Find the excluded observation.

Answer 1

Since mean of 4 observation = 20
⇒ Sum of 4 observation 
= 4 x 20
= 80
By excluding one observation,
the mean of the remaining 3 observations = 15
⇒ Sum of remaining 3 observation
= 3 x 15
= 45
∴ Excluded observation
= Sum of 4 observation - Sum of 3 observations
= 80 - 45
= 35.