Age Puzzle

#151

There are 10 members in a Muslim family. Their average age is "X" years. 
It remain same after 4 years, because 1 young child is added to the family replacing an old member. 
Find the age difference between the child and the old one.... 

  Difficulty Level - 3/5

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  Solution : 

   36 [ 10 * X + (40 - 4) + A - B = 10 * X ] 
{ 10 * X is Sum of ages of members ; (40 - 4) is age increased in 4 years ; A is age of small child ; B is age of old member }