Explanation

• The son's current age be $x$
  
• The man's current age is 5 times the son's age, so the man's age is $5x$.

• Four years ago, the product of their age was 52.

Four years ago:

• The son's age was $x - 4$.
• The man's age was $5x-4$.

According to the problem, the product of their ages 4 years ago is 52:

$$(x-4)(5x-4)=52$$

1. Expand the left side: