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
So, the answer is