The simplified expressions are as follows:
- \(\frac{\sqrt{5 x^{7}}}{\sqrt{3 x}} \cdot \sqrt{15 x^{2}} = 5 x^{4}\)
- \(5 x^{4}\) (already simplified)
- \(5 x^{2} \sqrt{x^{6}} = 5 x^{5}\)
- \(\sqrt{25 x^{8}} = 5 x^{4}\)
- \(25 x^{8}\) (already simplified)
Thus, the answers are:
- \(\boxed{5 x^{4}}\)
- \(\boxed{5 x^{4}}\)
- \(\boxed{5 x^{5}}\)