$\begin{align}\text{At present, father's age = x years}\\ \text{& son's age = y years} \end{align}$
$\begin{align}\text{∴ 4 years ago, father's age would be (x-4) years} \\ \text{ & son's age would be (y-4) years} \end{align}$
Now, given that,
Four years ago, the father's age was three times the age of his son.
$∴ (x-4) = 3(y-4) ---------- 1)$
Also given that,
The total of the ages of the father and the son $\underline{\text{after four years}}$ will be 64 years.
$∴ ( x+4 )+ (y+4 ) = 64 \\Or, x = 64 - 8 - y \\ Or, x = 56 - y ------ 2)$
Now, putting the $x's$ value in the equation $1)$
$ (x-4) = 3(y-4) \\ Or, 56-y-4 = 3y - 12 \\ Or, 52-y = 3y-12 \\ Or, 52+12 = 3y+y \\ Or, 4y = 64 \\ Or, y = \dfrac{64}{4} = 16$
$\color{maroon}{\text{∴ Son's present age is 16 years}}$
Now, putting the value of $y = 16$ in equation $2)$, we get -
$x = 56 - y \\ = 56 - 16 \\ = 40$
$\color{green}{\text{∴ Father's present age is 40 years. }}$