1. **State the problem:** A man is 4 times as old as his son. The difference between their ages is 21. Find the son's age. 2. **Set variables:** Let the son's age be $x$. 3. **Express the man's age:** Since the man is 4 times as old, his age is $4x$. 4. **Write the equation for the difference:** The difference between their ages is 21, so: $$4x - x = 21$$ 5. **Simplify the equation:** $$3x = 21$$ 6. **Solve for $x$:** $$x = \frac{21}{3}$$ 7. **Cancel common factors:** $$x = \frac{\cancel{21}}{\cancel{3}} = 7$$ 8. **Interpret the result:** The son's age is 7 years. **Final answer:** 7