$A$ random variable $X$ follows the binomial distribution and $X \sim B(n, 0.3)$. If the mean of $X$ is three times as large as the standard deviation of $X$,then $n=$

$A$ fair coin is tossed a fixed number of times. If the probability of getting five heads is equal to that of getting seven heads, then the probability of getting four heads is

$A$ fair coin is tossed $99$ times. If $X$ is the number of times head occurs,then $P[X=r]$ is maximum when $r=$

$A$ die is thrown $100$ times. If the success is defined as getting an even number,then the variance of the number of successes is:

$A$ student writes an examination which contains $8$ true or false questions. If he answers $6$ or more questions correctly,he passes the examination. If the student answers all the questions,then the probability that he fails in the examination is

$A$ multiple choice test consists of $5$ questions,each question having $4$ responses. There is only one correct response and the remaining $3$ are incorrect responses. If a candidate attempts all the $5$ questions,then the probability that he answers at least $3$ questions incorrectly is