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.
He is 29. If the oldsters end up winning, Federer would become the oldest man to win Wimbledon in the Open era, which began in 1968. Serena would become not only the oldest woman to win Wimbledon, but to win any Grand Slam tournament in the Open era.