Haven't taken physics but I would assume if her friend is standing in front of her that you would add up the speeds and get 30 km/hr.
When boat is sunk into the liquid the net buoyancy on the boat is counterbalanced by weight of the boat
So here weight of the boat = Buoyancy force
let say boat is sunk by distance "h"
now we can say


now by above force balance equation we can write




so boat will sunk by total 5 mm distance
Answer:

Explanation:
The frequency of a sodium vapor lamp, 
We need to find the wavelength of light. The relation between wavelength, frequency and speed of light is given by :

So, the wavelength of light is equal to
.