C

Find the surface area of a right pyramid whose base is a regular hexagon each side of which is $6\mathrm{cm}$ and height $9\mathrm{cm}$ (see the picture).

A plane is flying at a speed of $900\mathrm{km}\cdot {\mathrm{h}}^{-1}$ and according to the compass the axis of the airplane is still heading westbound. What angle will the flight path of the airplane make with the east-west direction if the south wind starts to blow at a speed of $10\mathrm{m}\cdot {\mathrm{s}}^{-1}$? Round the result to two decimal places.

Calculate the height between two floors if you know that the number of stairs between the floors is $16$, the slope of the staircase is ${30}^{\circ }$ and the depth of a stair is $25\mathrm{cm}$.

An observer was watching an approaching plane flying at a height of $3000\mathrm{m}$ in a straight line with constant velocity. At the first moment of measurement the observer saw the plane to be at an angle of elevation of ${25}^{\circ }$. After $10$ seconds the angle of elevation changed to ${35}^{\circ }$. What was the speed of the plane? Round the result to ones.