 Introduction
 Experimental Probability
 Range of Probability Measure
 Probability Space
 Theoretical Probability
 Types of Probability
 Tree Diagram
 Past KCSE Questions on the Topic.
Introduction
 The likelihood of an occurrence of an event or the numerical measure of chance is called probability.
Experimental Probability
 This is where probability is determined by experience or experiment. What is done or observed is the experiment. Each toss is called a trial and the result of a trial is the outcome.
 The experimental probability of a result is given by (the number of favorable outcomes)
(the total number of trials)
Example
A boy had a fair die with faces marked 1 to6 .He threw this die up 50 times and each time he recorded the number on the top face. The result of his experiment is shown below.
face  1  2  3  4  5  6 
Number of shown up 
11  6  7  9  9  8 
What is the experimental provability of getting?
 1
 6
Solution
 P(Event) = the number of favorable outcomes
the total number of trials
P(1)= ^{11}/_{50}  P(4)= ^{9}/_{50}
Example
From the past records, out of the ten matches a school football team has played, it has won seven.How many possible games might the school win in thirty matches?
Solution
P(winning in one math) = ^{7}/_{10.}
Therefore the number of possible wins in thirty matches = ^{7}/_{10} x 30 = 21 matches
Range of Probability Measure
 If P(A) is the probability of an event A happening and P(A') is the probability of an event A not happening,
Then P(A')= 1 − P(A) and P(A') + P(A)= 1  Probability are expressed as fractions, decimals or percentages.
Probability Space
 A list of all possible outcomes is probability space or sample space.
 The coin is such that the head or tail have equal chances of occurring.
 The events head or tail are said to be equally likely or equiprobable.
Theoretical Probability
 This can be calculated without necessarily using any past experience or doing any experiment.
 The probability of an event happening = number of favorable outcomes
total number of outcomes
Example
A basket contains 5 red balls, 4 green balls and 3 blue balls. If a ball is picked at random from the basket, find:
 The probability of picking a blue ball
 The probability of not picking a red ball
Solution
 Total number of balls is 1 2
The number of blue balls is 3
therefore, P (a blue ball) =^{3}/_{12}  The number of balls which are not red is 7.
Therefore P (not a red ball)= ^{7}/_{12}
Example
A bag contains 6 black balls and some brown ones. If a ball is picked at random the probability that it is black is 0.25. Find the number of brown balls.
Solution
Let the number of balls be x
Then the probability that a black ball is picked at random is ^{6}/_{x}
Therefore ^{6}/_{x} = 0.25
x = 24
The total number of balls is 24
Then the number of brown balls is 24 − 6 =18
Note:
 When all possible outcomes are countable, they are said to be discrete.
Types of Probability
Combined Events
 These are probability of two or more events occurring
Mutually Exclusive Events
 Occurrence of one excludes the occurrence of the other or the occurrence of one event depends on the occurrence of the other.
 If A and B are two mutually exclusive events, then ( A or B) = P (A) + P (B). For example when a coin is tossed the result will either be a head or a tail.
Example
If a coin is tossed ;
P(head) + P( tail)
= ^{1}/_{2} + ^{1}/_{2} = 1
Note;
 If [OR] is used then we add
Independent Events
 Two events A and B are independent if the occurrence of A does not influence the occurrence of B and vice versa.
 If A and B are two independent events, the probability of them occurring together is the product of their individual probabilities .That is;
P (A and B) = P (A) x P(B)
Note;
 When we use [AND] we multiply ,this is the multiplication law of probability.
Example
A coin is tosses twice. What is the probability of getting a tail in both tosses?
Solution
The outcome of the 2nd toss is independ of the outcome of the first .
Therefore;
P (T and T ) = P( T) X P( T)
= ^{1}/_{2 }x ^{1}/_{2 }= ^{1}/_{4}
Example
A boy throws fair coin and a regular tetrahedron with its four faces marked 1,2, 3 and 4. Find the probability that he gets a 3 on the tetrahedron and a head on the coin.
Solution
These are independent events.
P (H) = ^{1}/_{2} , P(3) = ^{1}/_{4}
Therefore;
P (H and 3) = P (H) x P (3)
= ½ x ¼
= ^{1}/_{8}
Example
A bag contains 8 black balls and 5 white ones. If two balls are drawn from the bag, one at a time,find the probability of drawing a black ball and a white ball.
 Without replacement
 With replacement
Solution
 There are only two ways we can get a black and a white ball: either drawing a white then a black,or drawing a black then a white. We need to find the two probabilities;
P(W followed by B) = P (W and B)
= ^{8}/_{13 }x ^{5}/_{12} = ^{10}/_{39}  P(B followed by W) = P (B and W)
= ^{5}/_{13} x ^{8}/_{12 }= ^{10}/_{39}Note; The two events are mutually exclusive, therefore.
P (W followed by B) or (B followed by W )= P(W followed by B ) + P (B followed by W)
= P (W and B) + P( B and W)
=^{40}/_{156} + ^{40}/_{156 }= ^{20}/_{39}Since we are replacing, the number of balls remains 13.
Therefore;
P (W and B) = ^{5}/_{13} x ^{8}/_{13} = ^{40}/_{169}P (B and W) = ^{8}/_{13} x ^{5}/_{13} = ^{40}/_{169}Therefore;
P [(W and B) or (B and W)] = P (W and B) + P (B and W)
=^{40}/_{169 }+ ^{40}/_{169}= ^{80}/_{169}
Example
Kamau ,Njoroge and Kariuki are practicing archery .The probability of Kamau hitting the target is 2/5,that of Njoroge hitting the target is ¼ and that of Kariuki hitting the target is ^{3}/_{7}, Find the probability that in one attempt;
 Only one hits the target
 All three hit the target
 None of them hits the target
 Two hit the target
 At least one hits the target
Solution
 P(only one hits the target)
=P (only Kamau hits and other two miss) =^{2}/_{5} x ^{3}/_{5} x ^{4}/_{7}
= ^{6}/_{35}
P (only Njoroge hits and other two miss) = ^{1}/_{4} x ^{3}/_{5} x ^{4}/_{7}
= ^{3}/_{35}P (only Kariuki hits and other two miss) = ^{3}/_{7} x ^{3}/_{5} x ¾
= ^{27}/_{140}P (only one hits) = P (Kamau hits or Njoroge hits or Kariuki hits)
= ^{6}/_{35} + ^{3}/_{35} + ^{27}/_{140}
= ^{9}/_{20}  P (all three hit) = ^{2}/_{5} x ^{1}/_{4} x ^{3}/_{7}
= ^{3}/_{70}  P (none hits) = ^{3}/_{5} x ^{3}/_{4} x ^{4}/_{7}
= ^{9}/_{35}  P (two hit the target ) is the probability of ;
Kamau and Njoroge hit the target and Kariuki misses = ^{2}/_{5} x ^{3}/_{7} x ^{4}/_{7}
Njoroge and Kariuki hit the target and Kamau misses = ^{1}/_{4} x ^{3}/_{7} x ^{3}/_{5}
Or
Kamau and Kariuki hit the target and Njoroge misses = ^{2}/_{5} x ^{3}/_{7} x ^{3}/_{4}
Therefore P (two hit target) = (^{2}/_{5} x ^{1}/_{4} x ^{4}/_{7}) + (^{1}/_{4} x ^{3}/_{7} x ^{3}/_{5}) + (^{2}/_{5} x ^{3}/_{7} x ^{3}/_{4})
= ^{8}/_{140} + ^{9}/_{140} + ^{18}/_{140}
= ¼  P (at least one hits the target) = 1 – P (none hits the target)
= 1 – ^{9}/_{35}
= ^{26}/_{35}
Or
P (at least one hits the target) = 1 – P (none hits the target)
= ^{26}/_{35}
Note;
 P (one hits the target) is different from P (at least one hits the target)
Tree Diagram
 Tree diagrams allows us to see all the possible outcomes of an event and calculate their probality.
 Each branch in a tree diagram represents a possible outcome .A tree diagram which represent a coin being tossed three times look like this;
 From the tree diagram, we can see that there are eight possible outcomes. To find out the probability of a particular outcome, we need to look at all the available paths (set of branches).
 The sum of the probabilities for any set of branches is always 1 .
 Also note that in a tree diagram to find a probability of an outcome we multiply along the branches and add vertically.
 The probability of three heads is:
P (H H H) = ^{1}/_{2} × ^{1}/_{2 }× ^{1}/_{2} = ^{1}/_{8}
P (2 Heads and a Tail) = P (H H T) + P (H T H) + P (T H H)
= ^{1}/_{2}× ^{1}/_{2} × ^{1}/_{2} + ^{1}/_{2} × ^{1}/_{2}× ^{1}/_{2}+ ^{1}/_{2} × ^{1}/_{2}× ^{1}/_{2}
= ^{1}/_{8} + ^{1}/_{8 }+ ^{1}/_{8}= ^{3}/_{8}
Example
Bag A contains three red marbles and four blue marbles.Bag B contains 5 red marbles and three blue marbles. A marble is taken from each bag in turn.
 What is the probability of getting a blue bead followed by a red
 What is the probability of getting a bead of each color
Solution
 Multiply the probabilities together
P(blue and red) = ^{4}/_{7} x ^{5}/_{8} =^{ 20}/_{56}
=^{5}/_{14}  P(blue and red or red and blue) = P( blue and red ) + P (red and blue)
= ^{4}/_{7} x ^{5}/_{8} + ^{3}/_{7} x ^{3}/_{8}
= ^{20}/_{56} + ^{9}/_{56}
=^{29}/_{56}
Example
The probability that Omweri goes to Nakuru is ¼ .If he goes to Nakuru, the probability that he will see flamingo is ½. If he does not go to Nakuru, the probability that he will see flamingo is ^{1}/_{3}. Find the probability that;
 Omweri will go to Nakuru and see a flamingo.
 Omweri will not go to Nakuru yet he will see a flamingo
 Omweri will see a flamingo
Solution
Let N stand for going to Nakuru ,N’ stand for not going to Nakuru, F stand for seeing a flamingo and F’ stand for not seeing a flamingo.
 P (He goes to Nakuru and sees a flamingo) = P(N and F)
= P(N) × P(F)
= ¼ × ½
= ^{1}/_{8}  P( He does not go to Nakuru and yet sees a flamingo) =P(N’) × P(F)
= P (N’ and F)
= ^{3}/_{4} × ^{1}/_{3}
= ¼  P ( He sees a flamingo) = P(N and F) or P ( N’ and F)
= P (N and F) + P (N’ and F)
= ^{1}/_{8} + ^{1}/_{4}
= ^{3}/_{8}
Past KCSE Questions on the Topic.
 The probabilities that a husband and wife will be alive 25 years from now are 0.7 and 0.9 respectively.
Find the probability that in 25 years time, Both will be alive
 Neither will be alive
 One will be alive
 At least one will be alive
 A bag contains blue, green and red pens of the same type in the ratio 8:2:5 respectively. A pen is picked at random without replacement and its colour noted
 Determine the probability that the first pen picked is
 Blue
 Either green or red
 Using a tree diagram, determine the probability that
 The first two pens picked are both green
 Only one of the first two pens picked is red.
 Determine the probability that the first pen picked is
 A science club is made up of boys and girls. The club has 3 officials. Using a tree diagram or otherwise find the probability that:
 The club officials are all boys
 Two of the officials are girls
 Two baskets A and B each contain a mixture of oranges and limes, all of the same size. Basket A contains 26 oranges and 13 limes. Basket B contains 18 oranges and 15 limes. A child selected a basket at random and picked a fruit at a random from it.
 Illustrate this information by a probabilities tree diagram
 Find the probability that the fruit picked was an orange.
 In form 1 class there are 22 girls and boys. The probability of a girl completing the secondary education course is 3 whereas that of a boy is ^{2}/_{3}
 A student is picked at random from class. Find the possibility that,
 The student picked is a boy and will complete the course
 The student picked will complete the course
 Two students are picked at random. Find the possibility that they are a boy and a girl and that both will not complete the course.
 A student is picked at random from class. Find the possibility that,
 Three representatives are to be selected randomly from a group of 7 girls and 8 boys. Calculate the probability of selecting two girls and one boy.
 A poultry farmer vaccinated 540 of his 720 chickens against a disease. Two months later, 5% of the vaccinated and 80% of the unvaccinated chicken, contracted the disease. Calculate the probability that a chicken chosen random contacted the disease.
 The probability of three darts players Akinyi, Kamau, and Juma hitting the bulls eye are 0.2, 0.3 and 1 .5 respectively.
 Draw a probability tree diagram to show the possible outcomes
 Find the probability that:
 All hit the bull’s eye
 Only one of them hit the bull’s eye
 At most one missed the bull’s eye

 An unbiased coin with two faces, head (H) and tail (T), is tossed three times, list all the possible outcomes.
Hence determine the probability of getting: At least two heads
 Only one tail
 During a certain motor rally it is predicted that the weather will be either dry (D) or wet (W). The probability that the weather will be dry is estimated to be ^{7}/_{10}. The probability for a driver to complete (C) the rally during the dry weather is estimated to be ^{5}/_{6}. The probability for a driver to complete the rally during wet weather is estimated to be ^{1}/_{10}. Complete the probability tree diagram given below.
What is the probability that: The driver completes the rally?
 The weather was wet and the driver did not complete the rally?
 An unbiased coin with two faces, head (H) and tail (T), is tossed three times, list all the possible outcomes.
 There are three cars A, B and C in a race. A is twice as likely to win as B while B is twice as likely to win as c. Find the probability that.
 A wins the race
 Either B or C wins the race.
 In the year 2003, the population of a certain district was 1.8 million. Thirty per cent of the population was in the age group 15 – 40 years. In the same year, 1 20,000 people in the district visited the Voluntary Counseling and Testing (VCT) centre for an HIV test. If a person was selected at random from the district in this year. Find the probability that the person visited a VCT centre and was in the age group 1 5 – 40 years.

 Two integers x and y are selected at random from the integers 1 to 8. If the same integer may be selected twice, find the probability that
 x – y = 2
 x – y is 5 or more
 x>y
 A die is biased so that when tossed, the probability of a number r showing up, is given by p(r)= Kr where K is a constant and r = 1 , 2,3,4,5 and 6 (the number on the faces of the die
 Find the value of K
 If the die is tossed twice, calculate the probability that the total score is 11
 Two integers x and y are selected at random from the integers 1 to 8. If the same integer may be selected twice, find the probability that
 Two bags A and B contain identical balls except for the colours. Bag A contains 4 red balls and 2 yellow balls. Bag B contains 2 red balls and 3 yellow balls.
 If a ball is drawn at random from each bag, find the probability that both balls are of the same colour.
 If two balls are drawn at random from each bag, one at a time without replacement, find the probability that:
 The two balls drawn from bag A or bag B are red
 All the four balls drawn are red
 The two balls drawn from bag A or bag B are red
 During inter – school competitions, football and volleyball teams from Mokagu high school took part. The probability that their football and volleyball teams would win were ^{3}/_{8 }and ^{4}/_{7} respectively. Find the probability that
Both their football and volleyball teams
At least one of their teams won  A science club is made up of 5 boys and 7 girls. The club has 3 officials. Using a tree diagram or otherwise find the probability that:
The club officials are all boys
Two of the officials are girls  Chicks on Onyango’s farm were noted to have either brown feathers brown or black tail feathers. Of those with black feathers ^{2}/_{3} were female while ^{2}/_{5 }of those with brown feathers were male. Otieno bought two chicks from Onyango. One had black tail feathers while the other had brown find the probability that Otieno’s chicks were not of the same gender was
 Three representatives are to be selected randomly from a group of 7 girls and 8 boys. Calculate the probability of selecting two girls and one boy
 The probability that a man wins a game is ¾. He plays the game until he wins. Determine the probability that he wins in the fifth round.
 The probability that Kamau will be selected for his school’s basketball team is ¼. If he is selected for the basketball team. Then the probability that he will be selected for football is ^{1}/_{3 }if he is not selected for basketball then the probability that he is selected for football is ^{4}/_{5}. What is the probability that Kamau is selected for at least one of the two games?
 Two baskets A and B each contains a mixture of oranges and lemons. Baskets A contains 26 oranges and 13 lemons. Baskets B contains 1 8 oranges and 15 lemons. A child selected a basket at random and picked at random a fruit from it. Determine the probability that the fruit picked an orange.
Download Probability  Mathematics Form 3 Notes.
Tap Here to Download for 50/
Get on WhatsApp for 50/
Why download?
 ✔ To read offline at any time.
 ✔ To Print at your convenience
 ✔ Share Easily with Friends / Students