arithmetic - the average age of a family of four members is 25 years if the age of the youngest member be 15 years then what was the average age of the family at the time of the birth of the youngest member - skillgun

At present the total age of the family = 4 * 25 = 100
The total age of the family at the time of the birth of the youngest member = [100-15-(15*3)]
= 40
Therefore, average age of the family at the time of birth of the youngest member = 40/3
= 13.33