The results showed that 25 percent of the girls' samples were mislabeled: half of the restaurant samples and six out of 10 grocery store samples.
If 50% of the restaurant samples were mislabeled and 60% of the grocery store samples were mislabeled how could the final result be 25%? Maybe they meant 50% of the restaurant samples were mislabled and 6 samples of all the samples taken from 10 grocery stores were mislabled? There were 60 samples altogether. If 18 of the samples were from restaurants and 42 were from 10 grocery stores then 15 of the 60 were mislabeled, or 25% of all samples.
But that isn't how the article reads to me.