Firstly, we need to find how many cups of butter is needed to bake

batches. We will need

cups of butter. This is

cups. Since we can only buy butter in boxes with 2 cups of butter in it, we will have to buy 3. This will give us 6 cups of butter, which will be enough. If we only buy 2, then we will only have 4 cups of butter, which won't be enough to bake 15 batches of cookies.
So, the answer is
