What is the official answer to this?
You applied the following reasoning:
Let us say H is no of hardcover copies sold after first 36,000 copies
The stem says no of paper back copies sold = 9*(H+36,000)
The question stem says the following:
From the time the first paperback copy was sold
until the last copy of the novel was sold. 9 times as many paperback copies as hardback copies were sold.
Looking at the bold portion in the question, I think the equation would be
36000 + H + 9H = 441000
This gives answer C.
Had the question said that the number of paperback copies were 9 times the TOTAL number of hard cover copies sold, your reasoning would be fine.
What do you think?