The finest surds ever!

Algebra Level 5

$\dfrac{(4+\sqrt{15})^{3/2}+(4-\sqrt{15})^{3/2}}{(6+\sqrt{35})^{3/2}-(6-\sqrt{35})^{3/2}}.$ If the value of above surd is equal to $\dfrac{a}{b}$ ,where $\text{gcd}(a,b)=1$ , find $a+b.$