The average age of a family of 5 members is 20 years. If the age of the youngest member is 10 years, what was the average age of the family at the time of the birth of the youngest member?

• At present the total age of the family = 5 * 20 = 100
  
  The total age of the family at the time of the birth of the youngest member = [100-10-(10*4)] = 50
  
  Therefore, average age of the family at the time of birth of the youngest member = 50/4 = 12.5.
• 11 years agoHelpfull: Yes(29) No(24)
• 100-10-(10*4). Why are we writing 10*4. I didn't get that. Please answer. Thanks.
• 8 years agoHelpfull: Yes(3) No(0)
• Since its average age of the family at the time of the birth of the youngest member we should decrease the age of all the member by 10 which is the current age of the youngest member . Hence 100(total age)-10(yungest mem age)-10*4(decreasing the age of other four members)=50 now taking average divided by 4 gives 12.5
• 8 years agoHelpfull: Yes(3) No(1)
• sum of age of family at present =100
  The total age of the family at time of youngest member's birth= 100-10-(10*4)
  =50
  Therefore avg age of family= 50/4=12.5
• 8 years agoHelpfull: Yes(3) No(0)
• 12.5 is the answer
  1)now the 5 members age average is 20.let the sum of their ages is x.
  2)(x/5) = 20
  3)x=100---------->sum of those 5 members age.
  4)at the time of the youngest person birth his/her age is 0.
  5)from now to last 10 years,every member in the family has lost 10 years each..
  6)so,100---->present sum of ages
  7)before 10 years their ages are(100-(10*4)-10)===50
  8)before 10 years there were only 4 members in the family even a child was born but his/her age is 0
  9)so 50/4=12.5
• 3 years agoHelpfull: Yes(0) No(0)