3. For what range of values of $a$ does the equation $\left|x^{2}-5 x\right|=$ $a$ have exactly two real roots?
(Tip: Let $y_{1}=\left|x^{2}-5 x\right|, y_{2}=a$. Then, the equation $\left|x^{2}-5 x\right|=a$ having exactly two real roots is transformed into the graphs of the functions $y_{1}=\left|x^{2}-5 x\right|$ and $y_{2}=a$ having exactly two intersection points. It is easy to know that the range of values for $a$ is $a>\frac{25}{4}$ or $a=0$.)