\(\left(5\sqrt{3}+3\sqrt{5}\right):\sqrt{15}\)
\(=\dfrac{5\sqrt{3}}{\sqrt{15}}+\dfrac{3\sqrt{5}}{\sqrt{15}}\)
\(=\dfrac{\sqrt{5}\sqrt{5}\sqrt{3}}{\sqrt{5}\sqrt{3}}+\dfrac{\sqrt{3}\sqrt{3}\sqrt{5}}{\sqrt{5}\sqrt{3}}\)
\(=\sqrt{5}+\sqrt{3}\)
`= 5 sqrt 3 : sqrt 15 + 3 sqrt 5 : sqrt 15`
`= sqrt 5 + sqrt 3`