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 Probability Questions

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FACTS  AND  FORMULAE  FOR  PROBABILITY  QUESTIONS

 

 

1. Experiment : An operation which can produce some well-defined outcomes is called an experiment.

 

2. Random Experiment :An experiment in which all possible outcomes are know and the exact output cannot be predicted in advance, is called a random experiment.

Ex :

i. Tossing a fair coin.

ii. Rolling an unbiased dice.

iii. Drawing a card from a pack of well-shuffled cards.

 

3. Details of above experiments:

i. When we throw a coin, then either a Head (H) or a Tail (T) appears.

ii. A dice is a solid cube, having 6 faces, marked 1, 2, 3, 4, 5, 6 respectively. When we throw a die, the outcome is the number that appears on its upper face.

iii. A pack of cards has 52 cards.

  • It has 13 cards of each suit, name Spades, Clubs, Hearts and Diamonds.
  • Cards of spades and clubs are black cards.
  • Cards of hearts and diamonds are red cards.

There are 4 honours of each unit. There are Kings, Queens and Jacks. These are all called face cards.

 

4. Sample Space: When we perform an experiment, then the set S of all possible outcomes is called the sample space.

Ex :

1. In tossing a coin, S = {H, T}

2. If two coins are tossed, the S = {HH, HT, TH, TT}.

3. In rolling a dice, we have, S = {1, 2, 3, 4, 5, 6}.

Event : Any subset of a sample space is called an event.

 

5. Probability of Occurrence of an Event : 

Let S be the sample and let E be an event.

Then, E⊆S

∴P(E)=n(E)n(S)

6. Results on Probability :

i. P(S) = 1    ii. 0≤P(E)≤1   iii. P(∅)=0

 

iv. For any events A and B we have : 

P(A∪B)=P(A)+P(B)-P(A∩B)

 

v. If A denotes (not-A), then P(A)=1-P(A)

Q:

If the standard deviation of a population is 3, what would be the population variance?

 

A) 9 B) 6
C) 8 D) 15
 
Answer & Explanation Answer: A) 9

Explanation:
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9 18666
Q:

An unbiased die is tossed.Find the probability of getting a multiple of 3.

A) 1/3 B) 1/2
C) 3/4 D) 3/2
 
Answer & Explanation Answer: A) 1/3

Explanation:

Here S = {1,2,3,4,5,6}

Let E be the event of getting the multiple of 3

Then, E = {3,6}

P(E) = n(E)/n(S) = 2/6 = 1/3 

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30 18194
Q:

If two dice are thrown together, the probability of getting an even number on one die and an odd number on the other is  ?

A) 1 B) 1/2
C) 0 D) 3/5
 
Answer & Explanation Answer: B) 1/2

Explanation:

The number of exhaustive outcomes is 36.
Let E be the event of getting an even number on one die and an odd number on the other. Let the event of getting either both even or both odd then = 18/36 = 1/2
P(E) = 1 - 1/2 = 1/2.

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17 18175
Q:

If a card is drawn at random from a pack of 52 cards,what is the chance of getting a spade or ace?

A) 0.25 B) 5/13
C) 0.20 D) 4/13
 
Answer & Explanation Answer: D) 4/13

Explanation:

Number of spades in a standard deck of cards=13
Number of aces in a standard deck of cards=4
And,one of the aces is a spade.
So, 13 + 4 - 1 = 16 spades or aces to choose from.
Therfore,probabiltiy of getting a spade or an ace=16/52=4/13

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18 17788
Q:

Two cards are drawn from a pack of well shuffled cards. Find the probability that one is a club and other in King

A) 1/52 B) 1/26
C) 1/13 D) 1/2
 
Answer & Explanation Answer: B) 1/26

Explanation:

Let X be the event that cards are in a club which is not king and other is the king of club.
Let Y be the event that one is any club card and other is a non-club king.
Hence, required probability:
=P(A)+P(B)

 

=12C1*1C152C2+13C1*3C152C2

 

=12*252*51+13*3*252*51= 24+7852*51 = 126

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27 17103
Q:

A coin is tossed 5 times. What is the probability that head appears an odd number of times?

A) 1/2 B) 1/3
C) 2/3 D) 1
 
Answer & Explanation Answer: A) 1/2

Explanation:

The possible outcomes are as follows :

5H, 5T, (H, 4T), (T, 4H), (2H, 3T) (3H, 2T), i.e. 6 outcomes in all.

Therefore the probability that head appears an odd number of times = 3/6 =1/2 (In only three outcomes out of the six outcomes, head appears an odd number of times).

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27 16655
Q:

A box contains 100 balls, numbered from 1 to 100. If three balls are selected at random and with replacement from the box, what is the probability that the sum of the three numbers on the balls selected from the box will be odd?

A) 1/6 B) 1/3
C) 1/2 D) 1/4
 
Answer & Explanation Answer: C) 1/2

Explanation:

P(odd) = P (even) =12 1(because there are 50 odd and 50 even numbers)

 

Sum or the three numbers can be odd only under the following 4 scenarios:

 

Odd + Odd + Odd = 12*12*12= 18

 

Odd + Even + Even = 12*12*12=18

 

Even + Odd + Even = 12*12*12=18

 

Even + Even + Odd = 12*12*12 = 18

 

Other combinations of odd and even will give even numbers. 

 

Adding up the 4 scenarios above:

 

= 18+ 18+18+ 18 = 48 = 12

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10 16461
Q:

In a party there are 5 couples. Out of them 5 people are chosen at random. Find the probability that there are at the least two couples ?

A) 6/7 B) 19/21
C) 7/31 D) 5/21
 
Answer & Explanation Answer: D) 5/21

Explanation:

Number of ways of (selecting at least two couples among five people selected) = (⁵C₂ x ⁶C₁)

As remaining person can be any one among three couples left.

Required probability = (⁵C₂ x ⁶C₁)/¹⁰C₅
= (10 x 6)/252 = 5/21

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12 15985