The average age of 7 members of a family is 35 years. If the youngest members are twins and they are 15 years old, then what was the average age (in years) of the family members at the time of the birth of the twins?

28

Present age of family of 7 members = 7 × 35 = 245 years

Total age of family at the time of birth of twins = 245 - 15 × 7 = 140 years

Average age of family at the time of twins birth = \(\frac{140}{5}\)

= 28 years