How do I get a question answered.

After watching the nested fractions video I have a question as to how he gets to a specific fraction.  He doesn't explain why all of the sudden  he gets b/b.  Can someone answer this for me.  the equation was 1/a-1/b divided by c and all of that divided by 1/d.  Right when he first starts I got lost on how he got to b/b.

If there is a way to ask questions during the videos, please let me know as I am new to the program.