A person writes 4 letters and addresses on 4 envelopes. If the letters are placed in the envelopes at random, the probability that not all letters are placed in correct envelopes is


Correct option is


Required probability

                                          = 1 – P (all the letters are put is correct envelops)

The numbers of ways of putting the letters in the envelops   


The number of ways of putting letters in correct envelops = 1



A pair of unbiased dice is rolled together till a sum of either 5 or 7 is obtained. The probability that 5 comes before 7 is


Each of two persons A and B toss three fair coins. The probability that both get the same number of heads is


The probability that a student is not a swimmer is 1/5. The probability that out of 5 students exactly 4 is swimmer is


Six distinct numbers are selected from first 150 natural numbers. The probability all the six numbers are divisible both by 3 and 5 is


There are two balls in an urn whose colours are not known (each ball can be either white or black). A white ball is put into the urn. A ball is drawn from the urn. The probability that it is white is


A natural number x is chosen at random from the first one hundred natural numbers. The probability that   is


An experiment has 10 equally likely outcomes. Let A and B two non – empty events of the experiment. If a consists of 4 outcomes, the number of outcomes that B must have so that A and B are independent, is


If AB and C are the events such that P(B) = 3/4, P(A ∩ B ∩ C’) = 1/3P(A’ ∩ B ∩ C’) = 1/3, then P (B ∩ C) is equal to


A fair coin is tossed n times. If the probability that head occurs 6 times is equal to the probability that occurs 8 times, then value of n is


A letter is known to have come from either TATANAGAR or CALCUTTA. On the envelope, just two consecutive letters, TA, are visible. The probability that the letter has come from CACUTTA is