The equation x² - 10x + c = 0 has two real solutions that differ by 4. What is the value of c?

Let the roots be $r$ and $r + 4$. For $x^2 - 10x + c = 0$, the roots add to 10, so $2r + 4 = 10$ and $r = 3$. The roots are 3 and 7, and $c$ is their product: $3 \times 7 = 21$. Check: $x^2 - 10x + 21 = (x - 3)(x - 7)$, and $7 - 3 = 4$. A common wrong answer is 25, from using the roots 5 and 5, which sum to 10 but do not differ by 4. Another is 10, the sum of the roots, which is the $x$-coefficient and not $c$. A third is 4, the difference.