What least value should be added to $2505$ so that it becomes a perfect square?

$16.02 \times 0.001 = ?$

Which of the following statement$(s)$ is/are $TRUE$?
$I.$ $\sqrt{144} \times \sqrt{36} \times \sqrt[3]{125} \times \sqrt{121} = 3960$
$II.$ $\sqrt{324} + \sqrt{49} < \sqrt[3]{216} \times \sqrt{9}$

What will come in the place of the question mark $(?)$ in the following expression?
$\sqrt{15^{2} \times 12 \div 9 - 125 + 21} = ?$

What should come in place of the question mark $(?)$ in the following equation?
$30.01^{2} - 19.98^{2} - ? = 21.81^{2}$

If $\left(x+\frac{1}{x}\right)^{2}=3$,then the value of $x^{206}+x^{200}+x^{90}+x^{84}+x^{18}+x^{12}+x^{6}+1$ is