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Explanation:

4 year back,

The average age of the family = 19 years

Sum or Total of all ages of all 4 family members = 19 x 4 =76 years

In the present day, that means after 4 years,

Sum or Total of present ages of these 4 family members = 76+(4x4) =92 years

Let the child’s present ages be K years.

Now the family is of 5 members(because a child has been added some years back)

Present average ages of family =

Sum of present ages of 4 members + Child’s present ages _________________________________________________

5

The average remains the same as it was 4 years back

-> 19 = (92 + K) / 5

-> K = 3 years = This is the present ages of the child

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