You are wrong (and you should realise you are wrong, because if you put Q = 20,000 in your equation, then you do not get a selling price of $100 !!).
The answer in the notes is correct, and I do explain why in the free lectures on this chapter.
(I assume that you are watching the lectures, because the notes are not to be used on their own. If you are not watching the lectures then you must buy a Study Text from one of the ACCA approved publishers, and study from that.)