The average age of a number of persons in a group was calculated as 35 years, which was 2.5 years more than the correct average as there was an error in recording the age of two persons as 38.5 years and 40 years instead of 29 years and 22 years respectively. The number of persons in the group was:

11

Let the number of persons in the group be x

Wrong sum of age of persons = 35x

Correct average age = 35 -  2.5 = 32.5

Correct sum of age of persons = 32.5x

⇒ 35x - 32.5x = 38.5 + 40 - 29 - 22

⇒ 2.5x = 27.5

⇒ x = 11