The key for a door is in a bunch of 10 keys. A man attempts to open the door by trying keys at random discording the wrong key. The probability that the door is opened in the fifth trial is

A person has a bunch of n keys , only one of which exactly fits a lock . The person tries to open the lock by trying the keys at random . The probability that he opens the lock is at the kth attempt on the assumption that he rejects the keys already tried is :

B_(1)B_(2) and B_(3) are three identical bulbs connected to a battery of steady e.m.f. with key K closed What happens to the brightness of the bulbs B_(1) and B_(2) when the key is opened ?