Questions about density

Why and how did the density of water change when salt was added to it?

The density of the water increases. The salt have a greater density than the water. So, when salt is added into the water, the density of the water increases.

Why did the egg sink in the tap water but float when salt is added?A

At the start, the density of the egg is bigger than the water, so it sank. But when salt is added, the density of the water increased. The density is then more than the egg. So the egg floats.