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.
We have shortlisted the favorite candidates to win the Champioships, Wimbledon, this year: Novak Djokovic. He is the current Wimbledon champion, and the first man since Rod Laver to hold the four Grand Slam titles at the same time. He has been the best tennis player during the last few years.