Thursday, January 16, 2014

Probability: Basic Understanding and Examples with solutions

Posted by Jasleen Behl
Today I'm going to discuss a very known concept Probability of Quantitative aptitude with you all. I hope you are well versed with this topic. I will try to teach you very easy method to solve problems related to this topic.

Probability means measuring the uncertainty. In other words, probability means how likely an event is to occur.

For example: I toss a coin. There will be two events: Head and Tail i.e. either head will occur or tail will occur. Therefore, both the events will have half probability.

Probability concept includes some terms. Firstly, go through these terms, then i will start discussing the basic concept.

Some basic terms

Random experiment

Experiment which does not give same results on performing repeatedly. The only factor is there should be homogeneous conditions.

Sample Space

While performing any experiment, all possible outcomes gives you a sample space. It is denoted by alphabet S.

For example: Tossing a coin. Sample space i.e. S = {H,T} 

Throwing a dice. Sample space will be S = {1,2,3,4,5,6}


 Any occurrence of an experiment is known as Event. Or I can say a subset of sample space. It is denoted as E.

For example: Occurrence of head, Occurrence of any odd number while throwing dice and so on.


If S is a sample space and E is occurrence of any event, then probability of occurrence of any event will be as follows:

P(E) = `(n(E))/(n(S))`

In simple words, Probability = Favorable Outcomes/ Total number of possible outcomes

Example1: An unbiased is thrown. What will be the probability of occurrence of a number multiple of `3`?

Solution: When a dice is thrown, numbers occurs are `1,2,3,4,5,6`. Therefore, sample space i.e. total number of possible outcomes will be:

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

Event = Occurrence of a number multiple of `3` i.e. `3` and `6`. Therefore, favorable outcomes are:

`E = {3,6}`

Therefore, Probability = Favorable outcomes/ Total number of possible outcomes

`P = 2/ 6 = 1/3`

Properties of Probability

Before start solving the questions, lets have a look on properties of probability. You should know about the basic properties of probability as it will help you to solve questions.
  • P(S) = `1` i.e. Probability  of total number of outcomes (Sample Space) is `1`  
  • 0 `le` P(E)`le` 1 i.e. Probability of Sample space lies between `0` and `1`.its never less than zero.
  • `barP(A) = 1- P(A)` i.e. Probability of not occurring of an event = 1- Probability of occurring an event.
  • If in any question, "and' is used for events, then this means we have to multiply the probabilities if events are independent i.e. P(A and B) = P(A) `times` P(B) 
  • Similarly, if or is used then both the probabilities will be added i.e. P(A or B) = P(A) + P(B)
Example2: A bag contains 8 red balls and 4 blue balls. Two balls are drawn at random. Find the probability of getting both balls of same color.

Solution: Total number of balls = `8 + 4= 12`

n(S)= number of ways of choosing `2` balls out of `12`= C`(12,2)`

`C(12,2) = (12!)/ (2! 10!)` = `66`

n(E)= number of ways of drawing same color balls i.e. either 2 out of 8 red balls OR 2 out of 4 blue balls 

`n(E) = C(8,2) + C(4,2) = 34`

Probability = `34/66` = `17/33`

Example3: One card is drawn from a pack of `52` cards. Find the probability that

i) card drawn is black
ii) card drawn is a queen
iii) card drawn is black and queen
iv) card drawn is either black or queen

Solution: n(S)= `52`

i) n(E) = n(black) = `26`

P(black) = `26/52` = `1/2`

ii) n(E) = n(queen) = `4`

P(queen) = `4/52` = `1/13`

iii) n(black and queen) = `2` ( two queens are of black color)

P(black and queen)= `2/52` = `1/26`

iv) n(black or queen) = `26` (black cards including `2` queen) + `2` (rest two queens)= `28`

P(black or queen) = `28/52` = `7/13`


  1. can you please post about, how to solve when three dice are tossed at the moment

  2. can you please post about, how to solve when three dice are tossed at the moment


I will try to respond asap