Suppose one of four similar appearing keys opens a lock. The keys are chosen successively until the correct one is found. If a key fails, it is put aside. What is the probability that 1, 2, 3, or 4...

Suppose one of four similar appearing keys opens a lock. The

keys are chosen successively until the correct one is found.

If a key fails, it is put aside. What is the probability that

1, 2, 3, or 4 trials are required?

Consider the case of n keys, one of which opens a lock.

What is the probability of 1, 2, ••• ,or n required trials

until success?