Bundles of 4 = 4x
unbundled apples = y

Total apple bought = 4x+y

So we have to make y minimum, Y is minimum at x = 5

y=0 and x=5

So the answer is 0

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?

If she purchases a bundles from the super market, the total number of apples she purchases from the super market = 4*a

Say, she purchases b apples from the convenience store

Total # of apples purchased = 4*a + b

For b to be minimum, a should be a multiple of 5,

=> 4*5 + 0 is a multiple of 5
=> 4*10 + 0 is a multiple of 5
=> 4*15 + 0 is a multiple of 5

So the least # of apples she must purchase in the convenience store is 0 (When purchase in bundles in multiples of 5 from the supermarket)

IMO:A

I interpreted this question wrong. Maybe I was over thinking it but the question asked how many single apples need to be purchased the ENSURE that the total number of apples is divisible by 5. So my logic was as follows:

If she bought:
1 bundle of 4 she would need to buy a single apple.
2 bundles, 2 single apples.
3 bundles, 3 single apples.
4 bundles, 4 single apples.

So if she buys 4 single apples, no matter how many bundles she purchases she can always make the total number of apples divisible by 5 even if she has to throw 3 or more away.

To me the word ensure means that she would buy as many single apples as needed, to make certain, that however many bundles she bought, she will have a total that is divisible my 5.

I even considered the OA but thought that it was too simple of an answer. To ensure that the total number of apples is divisible by 5 just make sure that the number of bundles is a multiple of 5.