In a village the average age of n people is 42 years But after the verification it was found that th...........

Explanation:

It is the same as a person with 20 years more age  replaces an existing person of the group ( or village)
Since the total age of the village having n persons, is being increased by 20 years and the average age of village is being increased by 1 year, hence there are total 20 people in the village.
Alternatively :  ( n x 42 ) + 20 = ( n x 43 )
=> n=20