4/5 of the original amount at the start of the month will remain. The amount at the start of every month will change. Thus:
an = 4/5 (an-1) ; where a1 = 500.
Answer:

Step-by-step explanation:
First, find the probability of him pulling out a book.
There are 11 objects in total, and there are 4 books.
So, the probability of pulling out a book is 
Next, find the probability of him pulling out a DVD after.
Since a book was taken out, there are only 10 objects left. There are 5 DVDs.
So, the probability of pulling out a DVD is
, or simplified to 
To find the probability that this happens in order, multiply the probabilities:
x 
= 
So, the probability is 