Problem Solving

There were 36.000 hardback copies of a certain novel sold before the paperback version was issued. 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. If a total of 441.000 copies of the novel were sold in all, how many paperback copies were sold?
(A) 45.000
(B) 360.000
(C) 364.500
(D) 392.000
(E) 396.900

let p be the number of paperback books sold after paperback becomes available:

p+(1/9)p = 441000-36000
10p/9 = 405000
p = 9*40500 = 364500

I think that this is a pretty simple problem, the only two mistakes that could be made is to solve for hardbacks instead of paperbacks, and to omit the fact that the 36,000 hardbacks that were solved preceded the release of the paperback version, which necessitates subtraction of the "old" hardbacks from the new total.

Otherwise, it's all about algebra.