If the container is one-eighth full before adding the water, it means that it is seven-eighths empty.
After adding 20 cups of water, the container is one-fourth empty, which means it is three-fourths full.
Therefore, the container went from being seven-eighths empty to three-fourths full, which is a change of 7/8 - 3/4 = 7/8 - 6/8 = 1/8.
Since 20 cups of water is 1/8 of the container's capacity, the container's total capacity is:
20 cups x 8 = 160 cups
To fill the empty container completely, we need an additional:
160 cups - 20 cups = 140 cups