Before I give you the answer, just a little tip for you that I learned is that if you can draw a picture then draw one. So how I would figure this out is by drawing a third of the bag for each day then adding them together. So day one 1/3 of the bag was eaten. Day two another 1/3 was eaten which would make it 2/3 and so on...by the end of the five days billy would've eaten 1 bag and 2/3 of another bag. Hope this helps!