a) $$\frac{5^{18} \cdot 5^2 \cdot 5^5}{5^{19}} = \frac{5^{18+2+5}}{5^{19}} = \frac{5^{25}}{5^{19}} = 5^{25-19} = 5^6 = 15625$$.
Ответ: $$15625$$