Question 697110
There has to be a typo. A father that young sound unreasonable, and the math results in ages that are not integers.
 
WHAT SHOULD HAVE BEEN:
The father's present age is {{{48}}} years.
That can be solved as follows:
{{{x}}} = age of the son 4 years ago
{{{x+4}}} = age of the son now.
The equation can be set up all at one as
{{{3(x+4)+(x+4)=48}}}
Going slower, to show the rationale:
The son's current age is {{{x+4}}} years.
The father says "I was 3 times as old as that when my son was born."
That means that the father was {{{3(x+4)=3x+12}}} when his son was born.
Since that was {{{x+4}}} years ago,
now the father must be {{{3x+12+(x+4)}}} years old.
{{{3x+12+(x+4)=3x+12+x+4=4x+16}}} and that equals {{{48}}}, so
{{{4x+16=48}}} --> {{{4x+16-16=48-16}}} --> {{{4x=32}}} --> {{{x=32/4}}} --> {{{highlight(x=8)}}}
  
THE PROBLEM AS POSTED
{{{x}}} = age of the son 4 years ago
{{{x+4}}} = age of the son now
The equation that results is
{{{3(x+4)+(x+4)=18}}}
It simplifies as before
{{{3(x+4)+(x+4)=18}}} --> {{{3x+12+(x+4)=18}}} --> {{{3x+12+x+4=18}}} -->{{{4x+16=18}}}
It solves as before
{{{4x+16=18}}} --> {{{4x+16-16=18-16}}} --> {{{4x=2}}} --> {{{x=1/2}}}
That would make the son {{{4&1/2}}} years old now.
But it does not make sense to say that the son was {{{1/2}}} year old four years ago (we would say he was just 6 months old)
While the son may proudly be saying "I am four and a half years old now,"
I would call that being just 4 years old, because age is usually stated as the age at the latest birthday, without fractions.
Furthermore, the father saying "I was 3 times as old when you were born" could mean
{{{3(4)=12}}} years old, or
{{{3(4&1/2)=13&1/2}}} years old (if we accept fractional ages),
and either way that would be too young.