I have a question on the following PS question that appeared on my MGMAT Online tests.

"Susie can buy apples from two stores: a supermarket that sells apples only in bundles of 4, and a convenience store that sells single, unbundled apples. If Susie wants to ensure that the total number of apples she buys is a multiple of 5, what is the minimum number of apples she must buy from the convenience store?"

A) 0 B) 1 C) 2 D) 3 E) 4

The answer explanation states that the minimum number of apples Susie can buy from the convenience stores is zero.

My assumption was that she 'MUST' buy apples from the convenience store, hence I did not choose zero.

Is this just a confusion on my side ?

Thanks in advance.
Michael

YES! The assumption you made was the culprit. Susie needn't buy apples from both the stores. Her goal is to get a multiple of 5.

So 0 is a multiple of 5.

This is always a source of my confusion. Please clarify.

I would say that 0 is a multiple of 5, because 5 x 0 = 0. But in this question, it doesn't matter, as the goal is for the total # of apples purchased to be a multiple of 5. So we can buy five 4-apple bundles from the supermarket and nothing from the convenience store and then the total # is 20, which is a multiple of 5.

Thanks for the explanation, the confusion from my side was my assumption that 'She must buy multiples of 5 apples AND she must buy from the convenience store'.

I really think that this is a very blurry question.
To me, it seems that it could be interpreted one way or another.

What is the minimum number of dollars you must pay me for answering your question?

Zero, of course! :) The term "minimum" generally refers to a non-zero numerical value. Zero falls into that category, hence it is a possible answer.

Even if she does not buy '0' apples, she can buy 20 apples (4x5) which is a multiple of 5, thereby avoiding buying any from convenience store.

sorry, but it can't. the problem statement follows the standard conventions of mathematical language, and is very precise. let me point you to the key parts:

Susie can buy apples from two stores: ...
the boldface words specifically imply that susie doesn't have to buy apples from both stores. in fact, they imply that she doesn't have to buy apples at all, if she doesn't want to.

If Susie wants to ensure that the total number of apples she buys is a multiple of 5
that includes zero. we know that's counterintuitive, but that counterintuitive quality is actually one of the main points of this problem!

what is the minimum number of apples she must buy from the convenience store?
notice, again, that this doesn't imply that she has to buy any apples. if the minimum that she must buy is zero (which it is, in fact), then she doesn't have to buy any apples at all.