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

A die is thrown. Describe the following events : $A$ : a number less than $7.$ , $B:$ a number greater than $7.$ Find the $A \cap B$

The probability of a sure event is

The probability that a teacher will give an unannounced test during any class meeting is $1/5$. If a student is absent twice, then the probability that the student will miss at least one test is

Two persons each make a single throw with a die. The probability they get equal value is ${p_1}$. Four persons each make a single throw and probability of three being equal is ${p_2}$, then

A bag $x$ contains $3$ white balls and $2$ black balls and another bag $y$ contains $2$ white balls and $4$ black balls. A bag and a ball out of it are picked at random. The probability that the ball is white, is