Give the step-by-step reasoning process and then the final answer. Mary uses plastic grocery bags that can hold a maximum of twenty pounds. She buys 4 pounds of green beans, 6 pounds milk, and twice the amount of carrots as green beans. How many more pounds of groceries can Mary fit in that bag?
First find the total weight of the carrots: 4 pounds * 2 = 8 pounds. Now add the total weight of each item: 6 pounds + 8 pounds + 4 pounds = 18 pounds. Now subtract the total weight of the groceries from the bag's weight limit: 20 pounds - 18 pounds = 2 pounds.
The final answer: 2.