Question: Committee of 4 people

You are breaking the task into 4 stages (select 1st student, select 2nd student, select 3rd student and select 4th student).

HOWEVER, when you do this, you should ask "Does the outcome of each stage differ from the outcomes of the other stages?" (for more on this, watch: https://www.gmatprepnow.com/module/gmat-counting/video/788)

In this case the answer is no; each selected person gets to be on the committee. That is, selecting A then D then E then B, is the SAME as selecting E then B then A then D

As such, we cannot use the stages approach. Instead, we must use combinations.

Here's my full solution: https://gmatclub.com/forum/gmatbuster-s-weekly-quant-quiz-10-ques-282406...

For some reason, the 5 posts above mine all have the same wrong solution. As you can see from my solution, we can distinguish the 4 boxes by naming them box #1, box #2, box #3, and box #4. So, there's no need to multiply our answer by 4! at the end (as all of the 5 other posts have done)

Cheers,
Brent

Question link: https://gmatclub.com/forum/a-certain-office-supply-store-stocks-2-sizes-...

If the notepads must be DIFFERENT sizes, then the question is impossible to solve.
There are only TWO sizes (say large and small)

In the original question, we're selecting groups of THREE notepads. Since there are only TWO different sizes (large and small), there's no way for all three notepads to be different sizes.

Does that help?