A superadditive function $f(\cdot)$ satisfies the following property

$$ f\left(x_1+x_2\right) \geq f\left(x_1\right)+f\left(x_2\right) $$

Which of the following functions is a superadditive function for $x>1$ ?

Five different books (P, Q, R, S, T) are to be arranged on a shelf. The books R and S are to be arranged first and second respectively from right side of the shelf. The number of different orders in which P, Q and T may be arranged is ________.

It would take one machine 4 hours to complete a production order and another machine 2 hours to complete the same order. If both machine work simultaneously at their respective constant rates, the time taken to complete the same order is _______ hours.

A 1.5 m tall person is standing at a distance of 3 m from a lamp post. The light from the lamp at the top of the post casts her shadow. The length of the shadow is twice her height. What is the height of the lamp post in meters?