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  1. Glossary
  2. /Mathematics
  3. /Probability

What is Probability?

In short

Probability is a number from 000 to 111 that measures how likely an event is. It equals the number of favourable outcomes divided by the number of all possible, equally likely outcomes.

In simple words

A probability of 000 means the event is impossible, and 111 means it is certain. The probabilities of an event and of its complement add to 111: P(not A)=1−P(A)P(\text{not } A) = 1 - P(A)P(not A)=1−P(A). For two events that cannot both happen, P(A or B)=P(A)+P(B)P(A \text{ or } B) = P(A) + P(B)P(A or B)=P(A)+P(B). For independent events, P(A and B)=P(A)×P(B)P(A \text{ and } B) = P(A) \times P(B)P(A and B)=P(A)×P(B).

Example

A fair die is rolled. The probability of an even number is 36=12\dfrac{3}{6} = \dfrac{1}{2}63​=21​. The probability of getting a number greater than 444 is 26=13\dfrac{2}{6} = \dfrac{1}{3}62​=31​.

Learn it properly

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Related terms

  • Permutations and Combinations
  • Mean, Median and Mode

← All glossary terms

Learn it in

  1. Probability: Equally Likely OutcomesClass 9 to 10
  2. Probability with Coins, Dice and CardsClass 9 to 10
All lessons on this topic →

Practise

  1. A leap year is chosen at random. Find the probability that it has 53 Sundays.Hard
  2. A spinner has 10 equal sectors numbered 1 to 10…Easy
A box holds 60 discs numbered 1 to 60. One disc is drawn at random…Medium
  • A bag holds 20 balls, some of them red. If 10 more red balls are put in, the probability…Medium
    • All problems on this topic →

    Quick questions

    1. Sunil tosses two coins and says: \"There are three results, two heads, one head or no…Hard
    2. Two fair dice are thrown. What is the probability that the sum of the numbers is a prime?Hard
    3. Cards numbered 2 to 101 are shuffled, and one is drawn. What is the probability that its…Hard

    More Mathematics terms

    1. Algebraic Identity
    2. Arithmetic Progression
    3. Complex Number
    4. Derivative
    5. Function
    6. Isosceles Triangle
    All Mathematics terms →

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