Square and Square Roots

Find the square root of: $$\displaystyle27\frac{9}{16}$$
If $$\left (x+\frac {1}{x}\right )=4$$, then $$\left (x^4+\frac {1}{x^4}\right )$$ is equal to
There are $$500$$ children in a school. For a P.T drill they have to stand in such a manner that the number of rows is equal is equal to number of columns .How many children would be left out in this arrangement? 
Find the square root of
$$1+{ a }^{ 2 }+{ \left( 1+{ a }^{ 2 }+{ a }^{ 4 } \right)  }^{ \frac { 1 }{ 2 }  }$$
Find $$k$$ if one root of the equation $$x^{2} - 6kx + 8 = 0$$ is twice the other.