problem: John is 31 years younger than his dad. The sum of their ages is 53 years. How many years old is John's dad?
solution: 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{align*}
  j &= d - 31 \\
  j + d &= 53 \\
  \end{align*}
  
  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.