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.
The other men's semifinal has Andy Murray taking on Roger Federer, with Murray at 11/4 and Federer at 4/1 on the updated odds. Federer has won Wimbledon seven times in his career, most recently in 2012 when he beat Murray in the final. Murray won in 2013.