The weight of an empty bucket can be seen in the graph as the point where the amount of sand is 0, as an empty bucket will have no sand in it.
That point, where Amount of sand is 0 lb, correspond to a Weight of the bucket of 1 lb:
Answer: Weight of the empty bucket: 1 lb.
We know it because an empty bucket is a bucket with 0 lb of sand, and the graph shows a weight of the bucket of 1 lb for 0 lb of sand.
