jmc

Let $j$ be John's age and $d$ be his dad's age. We are trying to find the value of $d$. We can create a system of two equations to represent the given information. They are

$$\begin{array}{rl}j& =d-31\\ j+d& =53\end{array}$$

We want to find $d$, so we need to eliminate $j$ from the equations above. Substituting the first equation into the second to eliminate $j$, we have $(d-31)+d=53$, or $d=42$. Thus, John's dad is $\boxed{42}$ years old.