GMAT Quantitative Reasoning Practice Test 7

Each one of the villagers, according to the question, has to own at least one of the two dogs.
Statement (1) is insufficient because it says nothing about the Bernard owners.
Statement (2) is insufficient because it says nothing about the Golden Retriever owners.
Combine the statements, all the information we need is present, 800−300−280 is equal to the number of people who own both races of dogs.