$$\dfrac{15}{18}=\dfrac{5\times 3}{6\times 3}=\dfrac{5}{6}$$
$$\dfrac{5}{6}=\dfrac{5}{6}\times \dfrac{2}{2}=\dfrac{10}{12}$$
$$\dfrac{5}{6}=\dfrac{5}{6}\times \dfrac{5}{5}=\dfrac{25}{30}$$.
Therefore, $$5, 12, 25$$ will come in the blanks respectively. Yes, all these are equivalent ratios.