50 students traveled to Europe, last year. Of these, 12 visited Amsterdam, 13 went to Berlin, and 15 were in Copenhagen. Some visited two cities: 3 visited both Amsterdam and Berlin, 6 visited Amsterdam and Copenhagen, and 5 visited Berlin and Copenhagen. But only 2 visited all three cities.
Question : How many students visited Copenhagen, but neither Amsterdam nor Berlin?
I have done the graph and I think the answer is 6 but I would like to learn how to compute it.