Page 265 - ICSE Math 7

Suppose Divya, Garima, Rohit, Ravi and Arman are the Try This
5 finalists out of which only 1 is to be selected to
represent the school in a competition. Each one of them Tell whether the following events is sure to
has the same chance of being selected. The probability happen, can happen but not sure or impossible.
of Divya being selected is 1 event out of 5 possible (a) Shreya is older than yesterday.
(b) A die when tossed shall land up with 6 on top.
outcomes. Therefore, the probability of Divya’s selection (c) Tomorrow will be a Sunny day.
1
1
is and is represented by P(Divya). The fraction (d) The Sun coming up from the west.
5 5 (e) A die when tossed shall land up on 8.
implies there is 1 chance of Divya’s selection out of a
total of 5. Similarly probability of a boy being selected to represent the school is denoted by
3
P(Boy) and is . The selection of Divya or a boy are events and the whole process of selection out
5
of 5 students is an experiment. Thus, probability can be considered a fraction expressing how many
times an event will occur in a given number of times.
The probability of an event E denoted by P(E) is given by:

P (E) = Number of outcomes in which E occurs
Total number of outcomes
Let’s try to understand the concept of probability using examples which result in outcomes having
equal chances of occurrence.
Example 1: A fair coin is tossed once. What is the probability of getting a head?
Solution: There are 2 possible outcomes, either head or tail. There
is only one outcome which is favourable. Hence, if we Maths Info
denote probability of head by P (H), then: A fair coin has equal chances

Favourable outcomes 1 of sharing up a head or a tail.
P (H) = =
Total outcomes 2
Example 2: A fair die is thrown once. Find the probability of getting the following on the die:
(a) 1 (b) an even number (c) a prime number
(d) number less than 7 (e) a number greater than 6
Solution: We know a die has six faces with equal chances of the numbers 1, 2, 3, 4, 5 or 6 coming
on top.

(a) Since 1 is the only favourable outcome
1
∴ P (1) =
6
(b) We have three even numbers, i.e., 2, 4 and 6

3 1
∴ P (even number) = =
6 2
(c) Since favourable outcomes are prime numbers, i.e., 2, 3 and 5
3 1
∴ P (prime number) = =
6 2
(d) Since each number on the die is less than 7

6
∴ P (number less than 7) = = 1
6

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