Question 1111879
.
<pre>
Let x = the age of the youngest child.


Then the next (= the middle) is (x+4);  the oldest is ((x+4)+2) years old,


and Mrs. Chen is (((x+4)+2)+20 years old, according to the condition.


Since the average of four is 39 now, the sum of the ages of the four is 39*4 = 156.


It gives you an equation

x + (x+4) + ((x+4)+2) + (((x+4)+2)+20) = 156


4x + 36 = 156

4x = 156 - 36 = 120  ====>  x = {{{120/4}}} = 30.


<U>Answer</U>.  The members of the family are  30, 34, 36 and 56 years old.


<U>Check</U>.   {{{(30+34+36+56)/4}}} = 39.   ! Correct !
</pre>

Solved.