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.
Wimbledon 2016: Novak Djokovic loses in shock upset to Sam Querrey. London: The unthinkable has occurred at Wimbledon: Novak Djokovic, the holder of all four grand slam titles, has been beaten in the third round. What world No.41 Sam Querrey started on Friday night before rain intervened, the American completed 7-6 (8-6), 6-1, 3-6, 7-6 (7-5). Extraordinary.