Final answer:
When using an extension ladder on a wall that is twenty feet high, the bottom of the ladder needs to be placed twenty feet away from the bottom of the wall.
Step-by-step explanation:
When using an extension ladder on a wall that is twenty feet high, the bottom of the ladder needs to be placed a distance away from the bottom of the wall that is equal to the height of the wall. Since the wall is twenty feet high, the bottom of the ladder should be placed twenty feet away from the bottom of the wall.