Awoman has 7 keys. only one of them will open her door. when seh arrives at the door there is no sufficient light for her to find the correct key. if she tries one key at a time and discards the keys that don't work, what is the probability that she will open the door with the first key she picks? what is the probability that it will take exactly three attempts to open the door? suppose that the first three attempts to open the door have been unsuccessful. what is the probability that she will pick the right key the fourth time?