If a person throws $3$ dice,what is the probability that the sum of the numbers obtained is exactly $15$?

If a coin is tossed $n$ times,then the probability that the head comes up an odd number of times is

Find the probability that when a hand of $7$ cards is drawn from a well-shuffled deck of $52$ cards,it contains exactly $3$ Kings.

From $3n$ consecutive integers, three integers are selected at random. The probability that their sum is divisible by $3$ is

If three cards are drawn randomly from a pack of $52$ playing cards, then the probability of getting exactly one spade card, exactly one king, and exactly one card having a prime number is

$A$ bag contains $a$ white and $b$ black balls. Two players $A$ and $B$ alternately draw a ball from the bag,replacing the ball each time after the draw,until one of them draws a white ball and wins the game. $A$ begins the game. If the probability of $A$ winning the game is three times that of $B$,then the ratio $a : b$ is