In a college of $300$ students, every student reads $5$ newspapers and every newspaper is read by $60$ students. The number of newspapers is

An experiment involves rolling a pair of dice and recording the numbers that come up. Describe the following events:

$A:$ the sum is greater than $8$,

$B : 2$ occurs on either die

$C:$ the sum is at least $ 7$ and a multiple of $3.$

Which pairs of these events are mutually exclusive ?

A fair coin is tossed repeatedly. If tail appears on first four tosses then the probability of head appearing on fifth toss equals

A coin is tossed. If it shows head, we draw a ball from a bag consisting of $3$ blue and $4$ white balls; if it shows tail we throw a die. Describe the sample space of this experiment.

Three letters are to be sent to different persons and addresses on the three envelopes are also written. Without looking at the addresses, the probability that the letters go into the right envelope is equal to