MathGenius, unfortunately, is incorrect. He would be right if you had 1 cup and spilled it, and had 3/8 left. However, you have another cup as well. MathGenius's answer does help explain why it works, though.
If you take 2 cups of flour, you can split it into two containers with 1 cup. Let's say that this was the case when you spilled the flour. You still have 3/8 of a cup in one container, and the other one is empty. As MathGenius said, 5/8 of a cup was spilled from the first container, but the entire other cup was spilled too. So the answer is 1 5/8.
