The least number to be subtracted from $16800$ to make it a perfect square is

What will come in the place of question mark $(?)$ in the following expression?
$561 \div 35.05 \times 19.99 = ?$

The value of $\frac{(81)^{3.6} \times (9)^{2.7}}{(81)^{4.2} \times 3}$ is

$4 \frac{1}{2} \times 4 \frac{1}{3} - 8 \frac{1}{3} + 5 \frac{2}{3} = ?$

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

What will come in place of the question mark (?) in the following expression?
$5 \frac{17}{37} \times 4 \frac{51}{52} \times 11 \frac{1}{7} + 2 \frac{3}{4} = ?$