Suppose the family consists of

Father who is 45
Mother who is 44
Jackie who is 19
Aaron  who is 17 
Bill   who is 15
Sarah  who is 10

Let's get their average age

45+44+19+17+15+10 = 150

Then 150 divided by 6 is 25.

So their average age is 25.

Now suppose the Father leaves.

Then there are only these 5 people left:

Mother who is 44
Jackie who is 19
Aaron  who is 17 
Bill   who is 15
Sarah  who is 10

Now let's get their average age

44+19+17+15+10 = 105

Then 105 divided by 5 is 21.

So their average age is 21.

That's the correct answer.  But there is a way to do the
problem without haveing to list the individual people and
their ages.  See if you can figure out how you could do
that without listing anybody or their ages and still get 21.

Edwin