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25 questions · self-marked practice — reveal the answer and mark yourself.

Question 12 Marks
There are four men and, six women on the city council. If one council member is selected for a committee at random, how likely is it that it is a woman?
Answer
Here total members in the council = 4 + 6 = 10
One member is selected out of 10 members
$\therefore \;n(S){ = ^{10}}{C_1} = 10$
Let A be the event that the member is a woman.
$n(A){ = ^6}{C_1} = 6$
Thus $P(A) = \frac{{n(A)}}{{n(S)}} = \frac{6}{{10}} = \frac{3}{5}$
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Question 22 Marks
The probability that a student will pass the final examination in both English and Hindi is 0.5 and the probability of passing neither is 0.1. If the probability of passing the English examination is 0.75. What is the probability of passing the Hindi examination?
Answer
Let A be the event that the student passes English examination and B be the event that the student passes Hindi examination.
Given, ${P(A\cap B)}$ = 0.5,
P(passing neither subjects) means P(not A and not B)
i.e., $P(\overline A\cap\overline B)$ = 0.1 and P(A) = 0.75.
Since, $\style{font-size:28px}{(\overline{A\cup B})=(\overline A\cap\overline B)}$ [by De Morgan's law]
$\therefore$ ${P(\overline{A\cup B)})=P(\overline A\cap\overline B)}$ = 0.1
${\Rightarrow1-P(A\cup B)}$ = 0.1
${\Rightarrow P(A\cup B)}$ = 1 - 0.1
${\Rightarrow P(A)+P(B)-P(A\cap B)}$ = 0.9
$\Rightarrow $ 0.75 + P(B) - 0.5 = 0.9
$\Rightarrow$ P(B) = 0.9 - 0.25
$\Rightarrow $ P(B) = 0.65
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Question 32 Marks
In an entrance test that is graded on the basis of two examination, the probability of a randomly chosen student passing the first examination is 0.8 and the probability of passing the second examination is 0.7. The probability of passing at least one of them is 0.95. What is the probability of passing both?
Answer
Let A be the event that the student passes the first examination
and B be the event that the student passes the second examination.
Given, P(A) = 0.8, P(B) = 0.7, and
Probability of passing at least A or B means $\style{font-size:28px}{P(A\cup B)}$
Given, $\style{font-size:28px}{P(A\cup B)=0.95}$
Since, $\style{font-size:28px}{P(A\cup B)=P(A)+P(B)- P(A\cap B)}$
So, $\style{font-size:28px}{P(A\cap B)=P(A)+P(B)- P(A\cup B)}$
= 0.8+0.7-0.95=1.5 - 0.95 = 0.55.
Hence, the probability of passing both examination is 0.55.
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Question 42 Marks
In class XI of a school 40% of the students study Mathematics and 30% study Biology, 10% of the class study both Mathematics and Biology. If a student is selected at random from the class, find the probability that he will be studying Mathematics or Biology.
Answer
Let A denotes the event that the student is studying Mathematics
and B denotes the event that the student is studying Biology.
Given, P(A) = 40% = $\frac{40}{100}$ , P(B)= $\frac{30}{100}$ and $\style{font-size:28px}{P(\;A\cap B\;)=10\%=\frac{10}{100}}$
Since, $\style{font-size:28px}{P(\;A\cup B\;)=P(A)+P(B)-P(A\cap B)}$
$\therefore$ Probability (A or B)
= $\style{font-size:28px}{P(\;A\cup B\;)=\frac{40}{100}+\frac{30}{100}-\frac{10}{100}}$ = $\style{font-size:28px}{\frac{40+30-10}{100}=\frac{60}{100}=\frac35}$
Hence the probability that the selected student will be studying Mathematics or Biology is $\frac35$
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Question 52 Marks
Events E and F are such that P (not E or not F) = 0.25 state whether E and F are mutually exclusive.
Answer
Given, P (not E or not F) = 0.25
$\style{font-size:28px}{\Rightarrow P\left(\overline E\cup\overline F\;\right)=0.25}$
$\style{font-size:28px}{\Rightarrow P\left(\;\overline{E\cap F}\;\right)=0.25}$$\style{font-size:28px}{\lbrack\;\because(\;\overline A\cup\overline B\;)\;=(\;\overline{A\cap B}\;)\;\;by\;De\;Morgan's\;law\;\rbrack}$
$\style{font-size:28px}{\Rightarrow1-P(\;E\cap F\;)=0.25}$
$\therefore$$\style{font-size:28px}{{P(\;E\cap F\;)=1-0.25=0.75}\neq\phi}$
Hence, E and F are not mutually exclusive events.
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Question 62 Marks
If E and F are events such that P(E) = $\frac{1}{4}$, P(F) = $\frac{1}{2}$ and P(E and F) = $\frac{1}{8}$. Find
(i) P (E or F) (ii) P (not E and not F)
Answer
Here P(E) = $\frac{1}{4}$, P(F) = $\frac{1}{2}$ and $P(E \cap F) = \frac{1}{8}$
(i) We know that $P(E \cup F) = P(E) + P(F) - P(E \cap F)$
$ = \frac{1}{4} + \frac{1}{2} - \frac{1}{8} = \frac{{2 + 4 - 1}}{8} = \frac{5}{8}$
(ii) P (not E and not F) $= P(\bar E \cap \bar F) = P(\overline {E \cup F} ) = 1 - P(E \cup F)$
$= 1 - \frac{5}{8} = \frac{{8 - 5}}{8} = \frac{3}{8}$
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Question 72 Marks
Given $P(A) = \frac{3}{5}$ and $P(B) = \frac{1}{5}$. Find P (A or B), if A and B are mutually exclusive events.
Answer
Here $P(A) = \frac{3}{5}\ , P(B) = \frac{1}{5}$
Since A and B are mutually exclusive events
$\therefore \;P(A \cup B) = P(A) + P(B)$
$\therefore \;P(A \cup B) = \frac{3}{5} + \frac{1}{5} = \frac{4}{5}$
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Question 82 Marks
A box contains 1 red, one 3 identical white balls. Two balls are drawn at random in succession without replacement. Write the sample space for this experiment.
Answer
The four balls in the box are R, W, W, W.
When two balls are drawn at random without replacement.
then the sample space is given by S = {RW, WR, WW}
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Question 92 Marks
One die of red colour, one of white colour and one of blue colour are placed, in a bag. One die is selected at random and rolled, its colour and the number on its upper most face is noted. Describe the sample space.
Answer
Let the dice of red colour, white colour and blue colour be denoted by R, W and B.
Faces of each die are marked 1, 2, 3, 4, 5 and 6.
$\therefore$ When a die is selected at random and rolled, then the sample space (S) of the experiment
is given by S= { R1, R2, R3, R4, R5, R6, W1, W2, W3, W4, W5, W6, B1, B2, B3, B4, B5, B6 }
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Question 102 Marks
2 boys and 2 girls are in a Room X and 1 boy and 3 girls in Room Y. Specify the sample space for the experiment in which a room is selected and then a person.
Answer
Let , in room $\mathrm{X}, \mathrm{B}_1, \mathrm{~B}_2$ denote 2 boys and $\mathrm{G}_1, \mathrm{G}_2$ denote 2 girls.
Also, let in room $Y, B_3$ denote the boy and $G_3, G_4, G_5$ denote 3 girls.
Hence the required sample space ( S )
can be given by $S=\left\{\mathrm{XB}_1, X \mathrm{B}_2, X \mathrm{G}_1, \mathrm{XG}_2, \mathrm{YB}_3, \mathrm{YG}_3, \mathrm{YG}_4, \mathrm{YG}_5\right\}$.
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Question 112 Marks
A coin is tossed and then a die is rolled only in case a head is shown on the coin.
Answer
When a coin is tossed either head or tail will turn up. When the head turns up then a dice is rolled otherwise not.
So, the total number of elementary events associated with this experiment is 1 + 6 $\times$ 1 = 7
The sample space,
S = {T, (H, 1), (H, 2), (H, 3), (H, 4), (H, 5), (H, 6)}
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Question 122 Marks
A coin is tossed and then a die is thrown. Describe the sample space for this experiment.
Answer
$\because$ When a coin is tossed, either tail or head will turn up, whereas when a dice is thrown, we have one face with either of 1,2,3,4,5 or 6.
So, the total number of elementary events associated with this experiment is $2\times  6 = 12$ and the sample space will be
S = {(H,1), (H,2), (H,3), (H,4), (H,5), (H,6), (T,1), (T,2), (T,3), (T,4), (T,5), (T,6) }
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Question 132 Marks
A coin is tossed four times. Describe the sample space for the indicated experiment.
Answer
Let H denote the event of a head and T denote the event of a tail.
Total number of elements in sample space =2 x 2 x 2 x 2 = 16 (because in single toss of coin sample space will be 2 and the coin is tossed 4 times)
We define the possible outcomes by an ordered set (w,x, y, z)
w denotes the event the coin is tossed for the first time
x denotes the event the coin is tossed for the second time
y denotes the event the coin is tossed for the third time
z denotes the event the coin is tossed for the fourth time
Sample space
S = {(H,H,H,H),(H,H,H,T),(H,H,T,H),(H,T,H,H),(T,H,H,H),(H,H,T,T),(H,T,H,T),(T,H,H,T),(T,H,T,H),(T,T,H,H),(H,T,T,H),(H,T,T,T),(T,H,T,T),(T,T,H,T),(T,T,T,H),(T,T,T,T)}
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Question 142 Marks
A die is thrown two times. Describe the sample space for the indicated experiment.
Answer
Let 1,2,3,4,5,6 denote the event the respective numbers comes when the die is thrown
The total number of sample space = (6 x 6) = 36
We define the possible outcomes by an ordered set (x, y)
where,
x denotes the digit occurring on the first die
y denotes the digit occurring on the second die
The sample spaces
S = {(1,1),(1,2),(1,3),(1,4),(1,5),(1,6),(2,1),(2,2),(2,3),(2,4),(2,5),(2,6),(3,1),(3,2),(3,3)(3,4),(3,5),(3,6),(4,1),(4,2),(4,3),(4,4),(4,5),(4,6),(5,1),(5,2),(5,3),(5,4),(5,5),(5,6),(6,1),(6,2),(6,3),(6,4),(6,5),(6,6)}
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Question 152 Marks
A die is thrown repeatedly until a six comes up. What is the sample space for this experiment?
Answer
In the given experiment,on throwing a die, six may come up on the first throw, the second throw,
the third throw, the fourth throw and so on till six comes up.
Hence, the sample space (S) of this experiment is given by,
S= {6, (1, 6), (2, 6), (3, 6), (4, 6), (5, 6), (1, 1, 6), (1, 2, 6), (1, 3, 6), (1, 4, 6), (1, 5, 6), (2, 1, 6), (2, 2, 6), (2, 3, 6), (2, 4, 6), (2, 5, 6), (3, 1, 6), (3, 2, 6), (3, 3, 6), (3, 4, 6), (3, 5, 6), (4, 1, 6), ... (4, 5, 6), (5, 1, 6), ... (5, 5, 6), ... }.
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Question 162 Marks
A coin is tossed. If it shows a tail, we draw a ball from a box which contains 2 red and 3 black balls. If it shows head, we throw a die. Find the sample space for this experiment.
Answer
When a coin is tossed then outcomes are $\mathrm{H}, \mathrm{T}$.
When the coin shows $T$ then a ball drawn from a box containing 2 red and 3 black balls,
then the outcomes are $R_1, R_2, R_1, R_2, R_3$
When the coin shows H then a die is thrown, then the outcomes are $1 ., 2,3,4,5,6$
Hence the required sample space $(\mathrm{S})$ is given by
$S=\left\{R_1, T R_2, T B_1, T B_2, T B_3, H 1, H 2, H 3, H 4, H 5, H 6\right\}$.
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Question 172 Marks
An experiment consists of rolling a die and then tossing a coin once if the number on the die is even. If the numbers on the die is odd, then coin is tossed twice. Write the sample space for this experiment.
Answer
When a die is rolled then outcomes are 1, 2, 3, 4, 5, 6.
On getting even numbers 2, 4, 6 on die, a coin is tossed once, then outcomes are H, T .
On getting odd numbers 1, 3, 5 on die, then a coin is tossed twice,
then the outcomes are HH, HT, TH, TT.
$\therefore$ The required sample space(S) is given by
S = {2H, 2T, 4H, 4T, 6H, 6T, 1HH, 1HT, 1TH, 1TT, 3HH, 3HT, 3TH, 3TT, 5HH, 5HT, 5TH, 5TT}.
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Question 182 Marks
The numbers 1, 2, 3 and 4 are written separately on four slips of paper. The slips are put in a box and mixed thoroughly. A person draws two slips from the box, one after the other without replacement. Describe the sample space for the experiment.
Answer
Here numbers 1, 2,3 and 4 are written separately on four slips of paper, put in a box and mixed thoroughly.
When two slips are drawn from the box without replacement then sample space is given by S = {(1, 2), (1, 3), (1, 4), (2,1), (2, 3), (2, 4), (3, 1), (3, 2), (3, 4), (4, 1), (4, 2), (4, 3)}
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Question 192 Marks
A coin is tossed. If the outcome is a head, a die is thrown. If the die shows up an even number, the die is thrown again. What is the sample space for the experiment?
Answer
When a coin is tossed then outcomes are H, T.
When the coin shows T, then there is no action.
When the coin shows H, a die is thrown then outcomes will be 1, 2, 3, 4, 5, 6.
When the die shows 1, 3, 5, then there is no action.
When the die shows 2, 4, 6, a die is thrown again and then outcomes are 1, 2, 3, 4, 5, 6.
Hence the required sample space (S) is given by,
S = {T, H1, H3, H5, H21, H22, H23, H24, H25, H26, H41, H42, H43, H44, H45, H46, H61, H62, H63, H64, H65, H66}
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Question 202 Marks
Suppose 3 bulbs are selected at random from a lot. Each bulb is tested and classified as defective (D) or non-defective (N). Write the sample space of this experiment.
Answer
Given that, defective bulbs are denoted by D and non-defective bulbs are denoted by N.
So the sample space when 3 bulbs are selected at random from a lot is given by
S - {DDD, DDN, DND, NDD, DNN, NDN, NND, NNN}
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Question 212 Marks
An experiment consists of tossing a coin and, then throwing it second time if a head occurs. If a tail occurs on the first toss, then a die is rolled, once. Find the sample space.
Answer
A coin is tossed then outcomes are H, T
If H comes again thown, then outcomes are H, T.
When a die is rolled then outcomes are 1, 2, 3, 4, 5, 6
Hence in this experiment the required sample space (S),
is given by S= {HH, HT, T1, T2, T3, T4, T5, T6}.
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Question 222 Marks
A coin is tossed three times. Describe the sample space for the indicated experiment.
Answer
A coin can either give a head or a tail
So, when 1 coin is tossed once the sample space = 2
when the coin is tossed 3 times sample space $=2 \times 2 \times 2=2^3=8$
The sample space can be found by the relative combination of the events
Let H denote the event of a head and T denote the event of a tail
The sample space can be found by the relative combination of the events
We define the possible outcomes by an ordered set (x, y, z)
where
x denotes the outcomes when the coin is tossed for the first time
y denotes the outcomes when the coin is tossed for the second time
z denotes the outcomes when the coin is tossed for the third time
Hence, the sample space is:
S = {(H,H,H) , (H,H,T) , (H,T,H) , (T,H,H) , (H,T,T) , (T,H,T) , (T,T,H) , (T,T,T)}
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Question 232 Marks
A coin is tossed three times, consider the events"
A: ‘No head appears’,
B: ‘Exactly one head appears’ and
C: ‘Atleast two heads appear’.
Do they form a set of mutually exclusive and exhaustive events?
Answer
Given that three coins are tossed. The sample space of the experiment is
S = {HHH, HHT, HTH, THH, HTT, THT, TTH, TTT}
Now, outcomes of event A = {TTT},
Outcomes of event B = {HTT, THT, TTH},
Outcomes of event C = {HHT, HTH, THH, HHH}
Now
Since A $\cup$ B $\cup$ C = {TTT, HTT, THT, TTH, HHT, HTH, THH, HHH} = S
Therefore, A, B and C are exhaustive events.
Also, A $\cap$ B = $\phi$, A $\cap$ C = $\phi$ and B $\cap$ C = $\phi$
Therefore, the events are pair-wise disjoint, i.e., they are mutually exclusive.
Hence, A, B and C form a set of mutually exclusive and exhaustive events.
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Question 242 Marks
Two dice are thrown and the sum of the numbers which come upon the dice is noted. Let us consider the following events associated with this experiment
A: the sum is even.
B: the sum is a multiple of 3.
C: the sum is less than 4.
D: the sum is greater than 11.
Which pairs of these events are mutually exclusive?
Answer
Two dice are thrown so there are 36 elements in the sample space S = {(x, y): x, y = 1, 2, 3, 4, 5, 6}.
Then
A = {(1, 1), (1, 3), (1, 5), (2, 2), (2, 4), (2, 6), (3, 1), (3, 3), (3, 5), (4, 2), (4, 4), (4, 6), (5, 1), (5, 3), (5, 5), (6, 2), (6, 4), (6, 6)}
B = {(1, 2), (2, 1), (1, 5), (5, 1), (3, 3), (2, 4), (4, 2), (3, 6), (6, 3), (4, 5), (5, 4), (6, 6)}
C = {(1, 1), (2, 1), (1, 2)} and
D = {(6, 6)}
For mutually exclusive events, there should be no element common,
We find that A $\cap$ B = {(1, 5), (2, 4), (3, 3), (4, 2), (5, 1), (6, 6)} $\ne$ $\phi$
Therefore, A and B are not mutually exclusive events.
Similarly A $\cap$ C $\ne$ $\phi$, A $\cap$ D $\ne$ $\phi$, B $\cap$ C $\ne$ $\phi$ and B $\cap$ D $\ne$ $\phi$.
Thus, the pairs of events, (A, C), (A, D), (B, C), (B, D) are not mutually exclusive events.
Also C $\cap$ D = $\phi$ and so C and D are mutually exclusive events.
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Question 252 Marks
Find the sample space associated with the experiment of rolling a pair of dice (one is blue and the other red) once. Also, find the number of elements of this sample space.
Answer
Since in each die the number of outcomes are 6.
So, the number of elements of this sample space is 6 × 6 = 36
Each outcome can be denoted by the ordered pair (x, y), where x is the number appeared on the blue die and y is the number appeared on the red die.
Therefore, this sample space is given by
S = {(x, y): x is the number on the blue die and y is the number on the red die}.
The sample space is given below:
S= {(1,1), (1,2), (1,3), (1,4), (1,5), (1,6), (2,1), (2,2), (2,3), (2,4), (2,5), (2,6), (3,1), (3,2), (3,3), (3,4), (3,5), (3,6), (4,1), (4,2), (4,3), (4,4), (4,5), (4,6), (5,1), (5,2), (5,3), (5,4), (5,5), (5,6), (6,1), (6,2), (6,3), (6,4), (6,5), (6,6)}
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