An unbiased die is tossed $500$ times. The standard deviation of getting sixes in these $500$ tosses is

For a binomial variate $X$ with $n=6$,if $P(X=2)=9 P(X=4)$,then its variance is

$A$ rifleman is firing at a distant target and has only $10 \%$ chance of hitting it. The least number of rounds he must fire to have more than $50 \%$ chance of hitting it at least once, is

$A$ fair coin is tossed a fixed number of times. If the probability of getting exactly $3$ heads equals the probability of getting exactly $5$ heads, then the probability of getting exactly one head is

$A$ box contains $15$ green and $10$ yellow balls. If $10$ balls are randomly drawn,one by one,with replacement,then the variance of the number of green balls drawn is

$A$ coin is tossed $n$ times. The probability of getting a head at least once is greater than $0.8$. Then, the least value of such $n$ is: