If average age of a five member group is same as three years ago. When a old man is replaced by a young man. What is the diff. between their ages.

• let the current age are = (younger)a,b,c,d,e(older)
  and average age= n = a+b+c+d+e/5
  average age 5 years ago = a+b+c+d+e-15/5
  As it given its equal to average age if old man replace by young man..
  so, a+b+c+d+e/5 = a+b+c+d+a-15/5
  =>a+b+c+d/5 + e/5 = a+b+c+d/5 + a-15/5
  => e/5 = a-15/5
  => e-a=15
  
• 12 years agoHelpfull: Yes(16) No(0)
• three years ago average is x and now also it is x but the younger person is replaced by older person it implies that answer = (No of persons)*time elapsed = 5*3 = 15.
  
  so answer is 15
• 12 years agoHelpfull: Yes(15) No(1)
• 12 years
  5*3=15 yrs
  4*3= 12 yrs
  
  refer this example-
  Let the old member be x
  Remaining member be y
  New member be z
  Average ages be a
  
  (x+9y)/10=a; --------------------(1)
  4 years later
  (9y +z+36)/10=a--------------(2) //36 because collective ages of 9 person 4 years after
  solving 1 and 2 you get
  
  You get z+36=x
  This means x is 36 year older then z
• 8 years agoHelpfull: Yes(0) No(1)