A liter of water weighs a kilogram so 15 mg/kg = 15 ppm on a mass over mass basis. If that is the flavor of ppm you want, done. But ppm can also be computed on a volume basis, molar ratio, etc. It is better tio use a clear form with units that identifies the basis such as mg/kg.
Serena Williams knocked off Lucie Safarova in the final of the women's draw at the French Open, and Bovada has her listed as the clear favorite on the odds to win Wimbledon this year; Williams sits as the 7/4 chalk on those betting lines.
2nd set: Williams 6-4 Muguruza* (*denotes most recent server) Williams won first set 6-4. Serena Williams is the Wimbledon champion! This ding-dong set swings the way of the American, who went 0-40 up in that game before Muguruza fired into the tramlines! Williams showing the power that was too much for Muguruza.