Azul when snorkeling and he noticed that the ratio of parrot fish to clownfish was 4: 3. he saw 20 parrot fish. how many clown fish did he see?

So Parrot Fish is the 4 and the clown fish was the 3. 

                               ---                                           ---

                               20                                            x

So what we need to do is find out what X is. 4 can go into 20 5 times... So multiply 3 with 5 to get 15. 

So Azul saw 15 clownfish.