Customs Officer Exam Prep

There are three dogs in a canine team in charge of inspecting cargo at an international airport for prohibited fruits, vegetables, and meats. Last week, one of these dogs identified 20 cartons of prohibited fruit among 120 total cartons on a flight arriving from overseas. Assuming that the dog selected the first carton to be inspected totally at random, what was the probability that this carton contained a shipment of prohibited fruit?