Let f and s be the ages of the father and the son. We have
[tex]\begin{cases}f+s=42\\fs=185\end{cases}[/tex]
From the first equation we derive
[tex]f=42-s[/tex]
Substitute this expression for f in the second equation and we have
[tex](42-s)s=185 \iff -s^2+42s-185=0 \iff s^2-42s+185=0[/tex]
The solutions to this equation are s=5 or s=37
Since the sum of the ages must be 42, the solutions would imply
[tex]s=5 \implies f=37,\quad s=37\implies f=5[/tex]
We can only accept the first solution, since the second would imply a son older than his father!
