Advertisements
Advertisements
Question
Three letters are dictated to three persons and an envelope is addressed to each of them, the letters are inserted into the envelopes at random so that each envelope contains exactly one letter. Find the probability that at least one letter is in its proper envelope.
Advertisements
Solution
Let the envelopes be denoted as A, B, C and the corresponding letters as a, b, c respectively.
(i) Ways of putting one letter in its proper envelope and the other two in the wrong envelopes
(Aa, Bc, Cb), (Ac, Bb, Ca), (Ab, Ba, Cc)
(ii) If two letters are put in the proper (correct) envelopes, then the third will also be in the proper (correct) envelope.
(iii) There is a way to put all the three letters in their proper (correct) envelopes (Aa, Bb, Cc).
Ways of putting letters in at least one proper envelope
3 + 1
= 4
Total ways of putting three letters in three envelopes = 3! = 6
∴ Probability of putting at least one letter in proper envelope = `4/6 = 2/3`
APPEARS IN
RELATED QUESTIONS
If `2/11` is the probability of an event, what is the probability of the event ‘not A’.
If E and F are events such that P(E) = `1/4`, P(F) = `1/2` and P(E and F) = `1/8`, find
- P(E or F)
- P(not E and not F).
A and B are events such that P(A) = 0.42, P(B) = 0.48 and P(A and B) = 0.16. Determine P(not A).
A dice is thrown. Find the probability of getting a multiple of 2 or 3.
In a simultaneous throw of a pair of dice, find the probability of getting a doublet of prime numbers
In a simultaneous throw of a pair of dice, find the probability of getting a doublet of odd numbers
In a simultaneous throw of a pair of dice, find the probability of getting a sum greater than 9
In a simultaneous throw of a pair of dice, find the probability of getting an even number on one and a multiple of 3 on the other
In a simultaneous throw of a pair of dice, find the probability of getting a sum more than 6
In a simultaneous throw of a pair of dice, find the probability of getting a sum less than 7
In a simultaneous throw of a pair of dice, find the probability of getting odd number on the first and 6 on the second
In a simultaneous throw of a pair of dice, find the probability of getting a number greater than 4 on each die
In a simultaneous throw of a pair of dice, find the probability of getting a total of 9 or 11
In a simultaneous throw of a pair of dice, find the probability of getting a total greater than 8.
Three coins are tossed together. Find the probability of getting at least one head and one tail.
Two dice are thrown. Find the odds in favour of getting the sum 4.
Two dice are thrown. Find the odds in favour of getting the sum What are the odds against getting the sum 6?
What are the odds in favour of getting a spade if the card drawn from a well-shuffled deck of cards? What are the odds in favour of getting a king?
A box contains 10 red marbles, 20 blue marbles and 30 green marbles. 5 marbles are drawn at random. From the box, what is the probability that all are blue?
A box contains 10 red marbles, 20 blue marbles and 30 green marbles. 5 marbles are drawn at random. From the box, what is the probability that at least one is green?
A box contains 6 red marbles numbered 1 through 6 and 4 white marbles numbered from 12 through 15. Find the probability that a marble drawn is even numbered
A box contains 6 red marbles numbered 1 through 6 and 4 white marbles numbered from 12 through 15. Find the probability that a marble drawn is red or even numbered.
Fill in the blank in the table:
| P (A) | P (B) | P (A ∩ B) | P(A∪ B) |
| 0.5 | 0.35 | ..... | 0.7 |
If A and B are two events associated with a random experiment such that
P(A) = 0.5, P(B) = 0.3 and P (A ∩ B) = 0.2, find P (A ∪ B).
The probability that a student will pass the final examination in both English and Hindi is 0.5 and the probability of passing neither is 0.1. If the probability of passing the English Examination is 0.75. What is the probability of passing the Hindi Examination?
One of the two events must occur. If the chance of one is 2/3 of the other, then odds in favour of the other are
The probability that a leap year will have 53 Fridays or 53 Saturdays is
A person write 4 letters and addresses 4 envelopes. If the letters are placed in the envelopes at random, then the probability that all letters are not placed in the right envelopes, is
If the probability of A to fail in an examination is \[\frac{1}{5}\] and that of B is \[\frac{3}{10}\] . Then, the probability that either A or B fails is
A bag contains 5 black balls, 4 white balls and 3 red balls. If a ball is selected randomwise, the probability that it is black or red ball is
An urn contains 9 balls two of which are red, three blue and four black. Three balls are drawn at random. The probability that they are of the same colour is
A box contains 6 nails and 10 nuts. Half of the nails and half of the nuts are rusted. If one item is chosen at random, the probability that it is rusted or is a nail is
