The blast made by the vessel's driver is telling you that he intends to pass by your left side. If you agree with his intention, you will respond by sounding one short blast in return. If you are not in agreement with this move or you do not really understand the intention of the driver, you will give the danger signal which is five short rapid blasts.
Mass never just disappears. The other 4kg had to go somewhere. It could have left the scene of the fire in the form of smoke particles and hot gases.
Vector resolution will be used to answer this question as the boat's velocity will have two components, one allowing it to travel across the river and the other taking it downstream.
