Boyle's Law

http://thescienceclassroom.org/chemistry-lessons/gases/the-gas-laws/

Boyle's Law is a fairly easy one. It stems from the normal law of gases, keeping temperature and moles constant. In the calculations, all you have to do is set the initial pressure times initial volume to equal the final pressure times final volume. This is a really easy law because the calculations are all in one line, so the relationship is easily identifiable.

https://en.wikipedia.org/wiki/Boyle%27s_law