517 * 539/555 =

$$\frac{5^{17} \cdot 5^{39}}{5^{55}} = \frac{5^{17+39}}{5^{55}} = \frac{5^{56}}{5^{55}} = 5^{56-55} = 5^1 = 5$$.

Ответ: $$5$$