Square and Square Roots

Divide:
$$21\sqrt {384}\div 8\sqrt {96}$$
Evaluate: [$$\displaystyle\sqrt{\left (5\, +\, 2\frac{21}{25}\right )\, \times\, \frac{0.169}{1.6}}$$] $$\times 100$$
Which one of the following cannot be the square of a natural number ?
Find a number that is divisible by 15, 18, and 25 and is a perfect square
On simplification the product of given expression $$\left (x - \dfrac {1}{x}\right )\left (x + \dfrac {1}{x}\right )\left (x^{2} + \dfrac {1}{x^{2}}\right )$$ is ________.