Answer:
18 years old
Solution:
Let x represent the age of the youngest child.
So the age of the oldest = 3x
If the ages of the two middle children are 19 and 23, and the average age of the four children is 28.5, let's go ahead and find x;
[tex]\begin{gathered} \frac{(x+19+23+3x)}{4}=28.5 \\ 4x+42=114 \end{gathered}[/tex]Let's go ahead and subtract 42 from both sides;
[tex]4x=72[/tex]Dividing both sides by 4, we'll have;
[tex]x=\frac{72}{4}=18[/tex]Therefore, the youngest is 18 years old.