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

This ratio is in the form of parrot fish:clown fish, with the relationship of 4:3, which means there are 4 parrot fish for every 3 clownfish.
To find how many clownfish he saw, you must create an equivalent ratio, with 20 in the place of parrot fish.
This is shown as 20:x, with x being how many clownfish he saw.

To find x, you must divide 20 by 4 then multiply that number by 3. Since the same thing must be done to both sides, whichever number was multiplied by 4 to get 20 had to have been multiplied by the 3 to get the value of x, otherwise it wouldn't be an equivalent ratio.

20 / 4 = 5
5 • 3 = 15

So the answer is that, with a ratio of 4 parrot fish:3 clownfish, if Azul saw 20 parrot fish, he saw 15 clownfish.
In ratio form, this is 20:15.