A bag contains four balls. Two balls are drawn randomly and found them to be white. The probability that all the balls in the bag are white is

If the coefficients $a$ and $b$ of a quadratic expression $x^2+a x+b$ are chosen from the sets $A=\{3,4,5\}$ and $B=\{1,2,3,4\}$ respectively, then the probability that the equation $x^2+a x+b=0$ has real roots is

A random variable $X$ has the following probability distribution

$$ \begin{array}{|c|l|l|l|l|l|l|l|l|} \hline \boldsymbol{X}=\boldsymbol{x} & 1 & 2 & 3 & 4 & 5 & 6 & 7 & 8 \\ \hline \boldsymbol{P}(\boldsymbol{X}=\boldsymbol{x}) & 0.15 & 0.23 & k & 0.10 & 0.20 & 0.08 & 0.07 & 0.05 \\ \hline \end{array} $$

For the events $E=\{x / x$ is a prime number $\}$ and $F=\{x / x<4\}$, then $P(E \cup F)=$

5 persons entered a lift cabin in the cellar of a 7 floor building apart from cellar. If each of them independently and with equal probability can leave the cabin at any floor out of the 7 floors beginning with the first, then the probability of all the 5 persons leaving the cabin at different floors is