If the mean and standard deviation of a binomial distribution are $20$ and $4$,respectively,then the number of trials is $.......$

$A$ coin is tossed $4$ times. The probability that at least one head turns up is

If the mean and variance of a binomial distribution are $4$ and $2$ respectively,then the probability of getting $2$ successes is:

$A$ random variable $X$ has a probability mass function $P(X=x) = \frac{{}^4C_x}{2^4}$ for $x = 0, 1, 2, 3, 4$. If $\mu$ and $\sigma^2$ are the mean and variance of the random variable $X$ respectively,then:

$A$ binomial random variable $X$ satisfies $9 \cdot P(X=4) = P(X=2)$ when $n=6$. Then $p$ is equal to

If $X \sim B(n, p)$ then $\frac{P(X=k)}{P(X=k-1)}=$