Susie can buy apples from two stores: a supermarket that

[Luci]

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 correct answer is A)-0 and here is the explanation:

Since the supermarket sells apples in bundles of 4, we can represent the number of apples that Susie buys from the supermarket as 4x, where x can be any integer ≥ 0. If the number of apples that Susie buys from the convenience store is simply y, the total number of apples she buys is (4x + y). We are asked to find the smallest possible value of y such that (4x + y) can be a multiple of 5.

We can solve this problem by testing numbers. Since the question asks us what is the minimum value for y such that (4x + y) can be a multiple of 5, it makes sense to begin by testing the smallest of the given answer choices. If y=0, can (4x + y) be a multiple of 5? Yes, because x could equal 5. (The value of (4(5) + 0) is 20, which is a multiple of 5.)

The correct answer is A.


I´m not very convinced with the explanation though... and I´m not convinced because the problem says she wants to ENSURE (this is what freaks me out) the total number of apples she buys is a multiple of 5.

Well the explanation it is clear, but does Susie ensures a multiple of 5? What if x=4....

I just don´t know what to say about this problem. I just put it here to see what the experts say.

Thanks

[Harish Dorai]

I encountered the same problem in my practice exam and I got it right. I reasoned it out like this:

I found how much closer to the target we can buy from the super market. Here the target number of apples to buy is a number that is a multiple of 5. So to find the max number we should buy from the supermarket, we need to find the multiple of 4 that is closest to the multiple of 5. And the answer to that is 20. Because 4 x 5 = 20. So in order to meet the requirement that the total number of apples is a multiple of 5, all we need to do is buy 5 bundles from the super market. Which means we need not buy anything from the convenience store. So answer is (A)

The problem doesn't say that it is mandatory to buy from both the convenience store and the supermarket. I agree that this problem is a little tricky.

[StaceyKoprince]

I agree that the wording is extra-tricky here (I don't like the "ensure" wording personally), but this set-up has been used on real GMAT questions, so we just have to deal with it! That's why we wrote it, in fact - so you could see it here first, when it doesn't count. :)