$A$ random variable $X \sim B(n, p)$. If the values of the mean and variance of $X$ are $18$ and $12$ respectively,then $n = $

If a random variable $X$ follows the Binomial distribution $B(33, p)$ such that $3P(X=0) = P(X=1)$,then the value of $\frac{P(X=15)}{P(X=18)} - \frac{P(X=16)}{P(X=17)}$ is equal to

Four cards are drawn successively with replacement from a well-shuffled deck of $52$ cards. The probability that exactly two cards are club cards is: (in $/128$)

$A$ die is thrown three times. Getting a $3$ or a $6$ is considered success. Then the probability of at least two successes is

Minimum number of times a fair coin must be tossed,so that the probability of getting at least one head,is more than $99 \%$ is

For an initial screening of an entrance exam,a candidate is given $50$ problems to solve. If the probability that the candidate can solve any problem is $\frac{4}{5}$,then the probability that he is unable to solve less than $2$ problems is: