In order to get at least one head with a probability $\ge 0.9$,the minimum number of times a coin needs to be tossed is:

Assuming that for a husband-wife couple,the chances of their child being a boy or a girl are the same,the probability of their two children being a boy and a girl is:

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

$A$ test consists of $6$ multiple choice questions,each having $4$ alternative answers of which only one is correct. The number of ways in which a candidate answers all $6$ questions such that exactly $4$ of the answers are correct is:

There are $6$ boxes labelled $B_1, B_2, \ldots, B_6$. In each trial,two fair dice $D_1, D_2$ are thrown. If $D_1$ shows $j$ and $D_2$ shows $k$,then $j$ balls are put into the box $B_k$. After $n$ trials,what is the probability that $B_1$ contains at most one ball?

In tossing $10$ coins,the probability of getting exactly $5$ heads is