$6.4 - 1.6 \div 0.2 = ?$

Simplified form of $\left[\left(\sqrt[5]{x^{-3 / 5}}\right)^{-5 / 3}\right]^{5}$ is

The value of $1-\frac{a}{1-\frac{1}{1+\frac{a}{1-a}}}$ is

The value of $\frac{2.75 \times 2.75 \times 2.75 - 2.25 \times 2.25 \times 2.25}{2.75 \times 2.75 + 2.75 \times 2.25 + 2.25 \times 2.25}$ is:

$\frac{3 \sqrt{2}}{\sqrt{6}-\sqrt{3}}-\frac{4 \sqrt{3}}{\sqrt{6}-\sqrt{2}}-\frac{6}{\sqrt{8}+\sqrt{12}}=?$

If $x + \frac{1}{x} = \sqrt{13}$,then $\frac{3x}{x^2 - 1}$ is equal to: