Why is it possible to use physical properties to separate a mixture

Using salt, water, and oil, by adding the salt you have eliminated the oil. then by boiling it the water evaporates, you take a funnel, gather the water vapor, put it in a cup. and everything is separated! so it does work.
 It is possible to separate a mixture because they are in pieces that are able for you to take away from the whole mixture. As for homogeneous mixtures, they can not separate. For example, kool-aid is a mixture you can not separate, mixing water and powder is impossible to separate when together.

The mass has escaped into a gas which you can not see so the law stays true.

You would do 1500/4 and the answer is 375