Page 3 - Lesson Notes-Probability

Solution
a. Let the probability of getting head is P(H)
Number of trials in which the head comes 49
P(H)= =
Total number of trials 100

b. Let the probability of getting tail is P(T)
Number of trials in which the head comes 51
P(T)= =
Total number of trials 100

c. The sum of two probability is
= P(H) + P(T)
= 49 + 51 = 1
100 100

Elementary Event
If there is only one possible outcome of an event to happen then it is called an
Elementary Event.
Remark:
If we add all the elementary events of an experiment then their sum will be 1.
49 51
In the above example P(H)= and P(T) =
100 100
P (H) + P (T) = 49 + 51 = 1
100 100
So, it is evident from the above example that the ‘Sum of all Elementary events
is equal to 1”
The general form
P (E) + P(E ̅ )= 1 (where E ̅ is ‘not E’).
P(E ̅) = 1− P(E)
P (E) and P(E ̅) are called complementary events.
Example-2
What is the probability of not hitting a six in a cricket match, if a batsman hits a
boundary six times out of 30 balls he played?
Solution
Let D be the event of hitting a boundary.
So, the probability of not hitting the boundary will be
P(D ̅) = 1 − 0.2
= 0.8
Impossible Events
While doing an experiment, if an event is not possible to occur then its probability will
be zero (0). This is known as an Impossible Event.
Example
You cannot throw a dice with number seven on it.

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