At a bakery, a baker prepares 6 dozen blueberry muffins for every 5 dozen apple muffins. if the baker prepares 16 dozen blueberry muffins, how many dozen apple muffins will he prepare? can you explain how to do this.

I would write a proportion, I can't use a fraction bar so they will just be on top of each other:
Blueberry Muffins    6
Apple Muffins          5
For every 6 Blueberry muffins there are 5 apple muffins (dozens)
So when he makes 16 blueberry muffins (dozens)
You write it next to the '6' because it's talking about blueberry muffin dozens.
The proportion now:

Blueberry Muffins  6   16
Apple Muffins        5    x
To find x, or the number of apple muffin dozens, you multiply 16 * 5, which is
80.
Then divide by 6.
80/6= 13.333
You can't have .333 of a dozen, so round it to 13.
***ANSWER*** 
He prepared 13 Apple muffin dozens.
Hope this helped <3