If $A$ and $B$ are two independent events,then $P(A/B) = $

From $21$ tickets numbered $1, 2, 3, \ldots, 21$,one ticket is drawn at random. Find the probability that the ticket drawn has a number divisible by $3$.

Four persons are selected at random from a group of $3$ men,$2$ women and $4$ children. The chance that exactly two of them are children is

Two dice are thrown together. The probability that at least one will show its digit $6$ is

An anti-aircraft gun can take a maximum of $4$ shots at an enemy plane moving away from it. The probabilities of hitting the plane at the first,second,third,and fourth shots are $0.4, 0.3, 0.2,$ and $0.1,$ respectively. What is the probability that the gun hits the plane?

An urn contains $10$ green,$12$ yellow,and $11$ white balls. What is the probability that when $2$ balls are selected,at least one is white?