Mutually Exclusive Event
Mutually exclusive events are events which cannot together at the same time. One event will pave way for the other, in such a case the separate probability are added together probabilities are added to give the combined probability.
Additional Law of Probability
If event A,B,C…. are mutually exclusive, the probability of A or B or C or…. Happening is the sum of their individual probabilities.
P(A) + P(B) + P(C) + ……..
Note: use the addition law to solve problems that contains the word or or either/or.
Worked Examples:
A bag contains 3 red balls, 4 blues balls 5 white balls and 6 black balls. A ball is picked at
Random, what is the probability that it is either:
- Red or blue
- Blue or black
- Red, white or blue
- Blue, white or black
- Neither red nor
Solution
P(R) = 3/18 P(B) = 4/18 P(W) = 5/18, P(BK) =6/18
(a)Pro(either red or blue) = 3/18 + 4/18 = 7/18
(b) Pro(blue or black) = 4/18 6/18
= 10/18 = 5/9
(c)P(red white or blue) = P(r) +P(w) + P(blue)
= 3/18 + 4/18 + 5/18
=12/18 = 2/3
(d) P(blue, white or black) = P(blue) + P(white) + P(black)
= 4/18 + 5/18 + 6/18
= 15/18 = 5/6
(e)P(neither Red or blue) = P(R or blue)1
= 1-P(R or blue)
= 1- 7/18
= 11/18
Worked Example 2
A letter is choosen at random from the word “COMPUTER” what the probability that it is
(a) either in the word cut or in the word ROPE
(b) neither in the word MET nor in the word UP?
Solution:
n(s) = 8
(a) P (either word CUT or ROPE) =
P (CUT) + P(ROPE)
= 3/8 + 4/8
= 7/8
(b) P(MET +UP)1 = 1- 5/8 = 3/8
Evaluation
F={2, 3, 7} and T = {10, 20, 30, 40}
(a) If one element is selected at random, from F, write down the probability that it is odd.
(b) If one element is selected at random from T, write down the probability that it is a multiple of 5
(c) If one element is selected at random, from FUT write down the probability of 42 or a multiple of 4
INDEPENDENT EVENT
Independent event are event which have no effect on each other. In such cases the
Separate probabilities are multiplied to give the combined probability.
Product Law
If event A, B, C is independent, the probability of A and B and C and …. Happening is
the product of their individual probabilities P(A) x P(B) x P(c) ……
Note: use the product law to solve problems that contains the words “and” or both/and
Worked Example: A coin is tossed and a die is then thrown what is the probability of getting a
head and a perfect square
Solution
P(H and perfect square)
P(H) = ½
(Perfect square = (1,4)
n (perfect square) = 2
n(s) = 6
P (perfect square) = 2/6 = 1/3
\ P (H and perfect square) = `1/2 x 1/3 =
=1/6
WorkedExample 2:
A bag contains 3 black balls and 2 white balls
(a) A ball is taken from the bag and then replaced, A second ball is chosen, what is the probability that
(i) They are both black
(ii) One is black and one is white
(iii) at least one is black
(iv) at most one is black
Solution.
With Replacement
i P(BB) = 3/5 x 3/5 = 9/25
ii Probabilities that one is black and one is white = P(BW) or P(WB)
P(one white one black) = P(BW) + P(WB)
= 3/5 x 2/5 + 2/5 x 3/5
P(BW) or P(WB) = 6/25 + 6/25 = 12/25
iii Prob ( at least one is black) = P( both are black) + P(one is black)
= P(BW) + P (WB) +P(BB)
= 12/25 + 9/25
= 21/25
iv At most one is black means either one is black or non is black i.e one is black or both are white.
P(at most one black) = P(BW) + P(WB) + P(WW)
= 6/25 + 6/5 + 4/25
= 16/25
GENERAL EVALUATION
1.A box contains ten marbles,seven of which are black and three are red.Three marbles are drawn one after the other without replacement.Find the probability of choosing a) one red,one black and one red marble(in that order).
b) two black marbles
c) at least two black marbles
d) at most two black marbles
WEEKEND ASSIGNMENT
Objectives
1. Two fair dice tossed together at once find the probability that the sum of the outcome is at least 10(a) 1/12 (b) 3/15 (c) 5/36 (d) 2/5
2 form a box containing 2 Red, 6 white and 5 black balls, a ball is randomly selected , what istheprobability that the selected ball is black.(a) 5/12 (b) 5/13 (c) 4/5 (d) 7/13
3. A bag contains 3 red, 4 black and 5 green identical balls, 2 balls are picked at random one after the other without replacement, find the prob that one is red and the other is green
(a) 5/22 (b) 7/23 (c) 15/132 (d) 12/13
4. A bag contains 3 white, 6 red and 5 blue identical balls, a ball is picked at random from the bag, what is the prob. that it is either white or blue? (a) 9/14 (b) 5/14 (c) 4/7 (d) 6/7
5. A bag contains red, black and green identical balls,a ball is picked andReplaced at 100 times.The table below shows the result of the 100 trails, What is the probability of picking a green ball.
| Colour | Red | Black | Green |
| No. of occurrence | 54 | 30 | 16 |
| (a)21/25 | (b)16 | (c)4/25 | (d)1/3 |
THEORY
1. A box contains 5 blue balls, 3 black balls and 2 red balls of the same size. A ball is selected at random, from the box and then replaced. A second ball is then selected, find the probability of obtaining.
(a) Two red balls
(b) Two blue balls or 2 black balls
(c) One black and one red ball in any order.
2. Solve the same problem if it is without replacement.
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