The average of 6 members in family is 22 years. If the age of the youngest is 7 years, what was average age of the members of the family at the time of birth of the youngest? 

Given Data:

The average age of 6 family members = 22 years.

Age of the youngest member = 7 years.

Concept:

The difference in total ages now and at the time of the
The youngest member's birth will be the total years elapsed times the number of members, and the average is the total age divided by the number of members.

Solution:

The current total age of the 6 family members is 6*22 = 132 years.

At the time of the youngest member's birth, 7 years ago, each member of the family was 7 years younger.

So the total age of the family members would have been:

132 - 7*6 = 132 - 42 = 90 years.

At that time, there were 5 members in the family (the youngest hadn't been born yet), so the average age would have been:

90/5 = 18 years.

 Therefore, the correct answer is 18 years.