NCERT Book Class 9 Mathematics Chapter 7 The Mathematics of Maybe: Introduction to Probability

The Mathematics of Maybe:
7 Introduction to Probability

7.1 What is Probability?
Probability is a type of measurement, similar to how we measure
quantities like length, area, or volume. However, instead of measuring
physical quantities, probability is used to measure the likelihood of
events. Specifically, it helps us express how confident or certain we are
that a particular event will occur. For example, you may ask your friend:
• Is it going to rain today?
• Will our school win the inter-school
hockey match tomorrow?
• Will I be chosen in the monthly lucky
draw to perform at the school assembly?
[The names of all the students in school
are written on slips of paper and one
slip is randomly selected.]
These events are examples of random
events. We know the possible outcomes
(either it will rain today or it will not; our school
team will either win, draw or lose the hockey
match; one student will be chosen to perform at the school assembly),
but we do not know in advance which one will definitely occur. That is,
there is an element of chance or randomness involved every time such
an event takes place.
Can we predict these outcomes with 100% certainty? One could
respond to these questions with words such as impossible or certain,
or using phrases such as less likely, more likely or equally likely. This
decision is based on different kinds of evidence that have been gathered.
For example, one friend might say, “The sun is shining brightly, so it’s
unlikely to rain today”, while another might observe, “It’s very hot, which
makes me think it could rain later”. In both cases, they are predicting
rainfall based on how they interpret the present weather conditions. This
is the subjective probability given to the event of today’s rainfall.
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