Simplify : $$\sqrt{[(12.1)^2-(8.1)^2]÷[(0.25)^2+(0.25)(19.95)]} $$

Find the value of   $$ {(0.75)^3\over1-.75}+[0.75+(0.75)^2+1]$$

$$ (17)^{2} + 6 × (13)^{2}-212×6$$

What mathematical operation should come at the place of ‘?’ in the equation:
2 ? 6 – 12 ÷ 4 +2 =11.

1-[5-{2+(-5+6-2)2}] is equal to 

$$\sqrt{x}÷ \sqrt{144} = 0.02, $$ then the value of x is 

Direction: what should come in place at question mark(?).

$$ {4\over 7}×{2{4\over 5}÷{2\over 5}=?}$$

The smallest natural number, by which 3000 must be divided to make the quotient a perfect cube, is :