If you go back to the original question, it also tells you they sold their entire stock...
Now you have to assume by inference that refers to stock levels of 1MB cards, (a) because that is the only size mentioned, and (b) because you are not told the stock levels of any other size card. Without the assumption that they ONLY have 1MB cards, a solution would be impossible !
EDIT ! I believe I have found a solution ! Just waiting confirmation from the OP....
From my perspective, it just becomes a question of what definitions you're willing to stretch, or what inferred assumptions you're willing to question.
for you point a) I've seen plenty of logic puzzles that have a red herring in there to trick you into making assumptions. The whole POINT of lots of brain teasers is to get you to make assumptions that prevent you from thinking outside the box.
for b) from my reading, you aren't told the stock level of any specific size card, just the total number of memory cards. Is it implied? sure! But to me it was also implied that money received is profit, not revenue, and that price is a single number, not a discount schedule.
I'm not saying your interpretation is wrong, I'm just saying that it's silly to disallow one interpretation when the other "correct" ones are justified with very similar arguments.