If out of $20$ consecutive whole numbers two are chosen at random, then the probability that their sum is odd, is
$\frac{5}{{19}}$
$\frac{{10}}{{19}}$
$\frac{9}{{19}}$
None of these
There are $n$ letters and $n$ addressed envelops. The probability that each letter takes place in right envelop is
Three integers are chosen at random from the first $20$ integers. The probability that their product is even, is
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
A box contains $15$ tickets numbered $1, 2, ....... 15$. Seven tickets are drawn at random one after the other with replacement. The probability that the greatest number on a drawn ticket is $9$, is
Dialing a telephone number an old man forgets the last two digits remembering only that these are different dialled at random. The probability that the number is dialled correctly, is