AAMC Archimede's Principle problem (MCAT 2015 Sample test)

Can someone please explain the Archimedes problem on the AAMC sample test?
The one asking about the density of a human body calculated by the weight in air and its weight submerged in water?

Answer was: Weight in air / (Weight in air - Weight in water)
I just really do not understand their explanation / the derivation for this.

Thanks