The mean and variance of 20 observations are found to be 10 and 4, respectively. On rechecking, it was found that an observation 9 was incorrect and the correct observation was 11. Then the correct variance is :
In a workshop, there are five machines and the probability of any one of them to be out of service on a day is . If the probability that at most two machines will be out of service on the same day is then k is equal to :
Let denote the complement of an event be any pair wise independent events with and then equal to: