$A$ bag contains $5$ brown and $4$ white socks. $A$ man pulls out two socks. The probability that these are of the same colour is

An urn contains $6$ yellow balls and $x$ black balls. When two balls are drawn at random,the probability that both are yellow is $\frac{5}{26}$,then the value of $x$ is

$A$ box contains $4$ defective and $6$ good machines. Two machines are selected at random without replacement. Find the probability that both the machines are good.

$A$ bag contains $5$ white,$7$ black and $4$ red balls. Three balls are drawn from the bag at random. The probability that all the three balls are white,is

If two numbers $a$ and $b$ are chosen from the set of integers $1$ to $39$,then the probability that those numbers satisfy the equation $7a - 9b = 0$ is

There are four machines and it is known that exactly two of them are faulty. They are tested,one by one,in a random order until both the faulty machines are identified. Then the probability that only two tests are needed is