Given,5 years ago average age of five persons = 20 years Formula: Average = Sum of all observations/total number of all observations Calculation: Sum of age of five persons 5 years ago =20×5=100 years Sum of present age of five persons = 100+5×5=125 years If a person of age 31 years joined the group, then sum of age of six persons = 125+31=156 ∴ Average age of six persons =