CBSE · NCERT · Class 9 Maths · Chapter 7

NCERT Solutions: Class 9 Maths Chapter 7 - The Mathematics of Maybe: Introduction to Probability

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Chapter-wise NCERT intext questions and exercise answers for The Mathematics of Maybe: Introduction to Probability, grounded in the official textbook.

Questions are taken verbatim from the NCERT textbook; answers were grounded against the chapter's content during generation. Items needing review are marked.
Sections in this chapter
Exercise 7.1 1Exercise 7.2 2Exercise 7.3 3Exercise 7.4 2Exercise 7.5 6
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1Exercise 7.11 questions
Q.1Rank the following events on a scale from 0 (Impossible) to 1 (Certain). Label each event: Impossible, less likely, equally likely (even chance), more likely, certain. Give reasons why you gave each event its ranking. (i) The next Monday will come after Sunday. (ii) It will snow in Mumbai in July. (iii) An elephant will walk through your classroom today. (iv) You will greet at least one friend at school tomorrow.v
Solution

Monday always follows Sunday in the calendar, so (i) is certain. Mumbai in July is hot and monsoon-season, not a place where snowfall normally occurs, so (ii) is impossible in ordinary school-level reasoning. An elephant entering a classroom is possible only in an unusual situation, so (iii) is less likely. Greeting a friend at school tomorrow is very likely if school is open and friends are present, but not logically guaranteed.

Answer:

(i) Certain; probability $1$.
(ii) Impossible or almost impossible in ordinary experience; probability close to $0$.
(iii) Less likely, practically close to $0$.
(iv) More likely; probability close to $1$, though not certain.

2Exercise 7.22 questions
Q.1A teacher mixes a large bag of sweets of different colours and randomly selects a sample of 30 sweets. She counts the number of sweets of each colour: 10 red sweets | 8 green sweets | 7 yellow sweets | 5 blue sweets. (i) Calculate the probability that a randomly picked sweet from the sample is green. (ii) If there are 600 sweets in total in the large bag, estimate how many are likely to be yellow, based on the sample results.v
Solution

In the sample, 8 out of 30 sweets are green, so the experimental probability of green is $\dfrac{8}{30}=\dfrac{4}{15}$. Yellow sweets form $\dfrac{7}{30}$ of the sample, so in 600 sweets the estimate is $\dfrac{7}{30}\times600=140$.

Answer:

(i) $\dfrac{8}{30}=\dfrac{4}{15}$
(ii) About $140$ yellow sweets.

Q.2A survey is conducted at a school where a random sample of 40 students is asked about their favourite club. The responses are: 14 students: Science Club | 11 students: Arts Club | 9 students: Sports Club | 6 students: Debate Club. Assume there are 800 students in the whole school. (i) What is the probability that a randomly chosen student from the sample prefers the Arts Club? (ii) Using the sample results, estimate how many students in the whole school are likely to prefer the Sports Club.v
Solution

Arts Club was preferred by 11 out of 40 students, so the probability is $\dfrac{11}{40}$. Sports Club was preferred by 9 out of 40 students, so the estimate for 800 students is $\dfrac{9}{40}\times800=180$.

Answer:

(i) $\dfrac{11}{40}$
(ii) About $180$ students.

3Exercise 7.33 questions
Q.1When a single 6-sided die is rolled, what is the total number of possible outcomes in the sample space?v
Answer:

There are $6$ possible outcomes: $\{1,2,3,4,5,6\}$.

Q.2For the following experiments write down the sample space S. (i) Rolling a die and tossing a coin together. (ii) Choosing a random integer between $-5$ and $+5$. (iii) A box containing 5 green and 7 red balls. One ball is drawn at random.v
Solution

For a combined die-and-coin experiment, pair each die outcome with each coin outcome. Between $-5$ and $+5$ inclusive, list every integer. For drawing one ball when only colour is recorded, the possible colour outcomes are green and red.

Answer:

(i) $S=\{(1,H),(1,T),(2,H),(2,T),(3,H),(3,T),(4,H),(4,T),(5,H),(5,T),(6,H),(6,T)\}$.
(ii) $S=\{-5,-4,-3,-2,-1,0,1,2,3,4,5\}$.
(iii) By colour, $S=\{\text{green},\text{red}\}$.

Q.3In a village fair, there are 3 popular snacks available: Samosa, Pakora, and Bhaji. For drinks, villagers can choose either Chai or Lassi. (i) List the sample space of all possible snack and drink combinations a person could choose at the fair. (ii) List the event 'Selecting Samosa as a snack.'v
Solution

Each snack can be paired with either of the 2 drinks, so there are $3\times2=6$ combinations. The Samosa event contains the two outcomes where the snack is Samosa.

Answer:

(i) $S=\{(\text{Samosa},\text{Chai}),(\text{Samosa},\text{Lassi}),(\text{Pakora},\text{Chai}),(\text{Pakora},\text{Lassi}),(\text{Bhaji},\text{Chai}),(\text{Bhaji},\text{Lassi})\}$.
(ii) Event $=\{(\text{Samosa},\text{Chai}),(\text{Samosa},\text{Lassi})\}$.

4Exercise 7.42 questions
Q.1There are two fruit baskets A and B. Basket A has one apple and two oranges. Basket B has one banana and one mango. You randomly pick one fruit from each basket. (i) Draw a tree diagram showing all possible pairs of fruits. (ii) List the sample space. (iii) What is the probability of picking one apple and one banana?v
Solution

Basket A has 3 fruits, of which 1 is an apple, so $P(\text{Apple})=1/3$. Basket B has 2 fruits, of which 1 is a banana, so $P(\text{Banana})=1/2$. The choices are independent, so the probability of apple and banana is $1/3\times1/2=1/6$.

Answer:

(i) The tree branches from Basket A to Apple or Orange, and from each of these to Banana or Mango.
(ii) By fruit type, $S=\{(\text{Apple},\text{Banana}),(\text{Apple},\text{Mango}),(\text{Orange},\text{Banana}),(\text{Orange},\text{Mango})\}$.
(iii) $P(\text{Apple and Banana})=\dfrac{1}{3}\times\dfrac{1}{2}=\dfrac{1}{6}$.

Q.2Let us say that you have a box containing 3 red pens, 4 black pens and 2 green pens. You pick a pen (without looking) from the box and put it back. Then your friend does the same. (i) What are the possible outcomes of the pen colours? Can you draw a tree diagram representing the possible outcomes? (ii) Can you use the tree diagram to guess the probability that both you and your friend pick pens of the same colour?v
Solution

Because the pen is replaced, both picks have the same colour probabilities: red $3/9$, black $4/9$, green $2/9$. Same colour can happen as RR, BB or GG. Add their probabilities: $\dfrac{3}{9}\cdot\dfrac{3}{9}+\dfrac{4}{9}\cdot\dfrac{4}{9}+\dfrac{2}{9}\cdot\dfrac{2}{9}=\dfrac{9+16+4}{81}=\dfrac{29}{81}$.

Answer:

(i) Colour-pair outcomes are $\{RR,RB,RG,BR,BB,BG,GR,GB,GG\}$, where R = red, B = black and G = green.
(ii) The probability of the same colour is $\left(\dfrac{3}{9}\right)^2+\left(\dfrac{4}{9}\right)^2+\left(\dfrac{2}{9}\right)^2=\dfrac{29}{81}$.

5Exercise 7.56 questions
Q.1Fill in the blanks. (i) The probability of an impossible event is _______. (ii) The set of all possible outcomes of a random experiment is called the __________. (iii) The probability of an event that is certain to happen is _______. (iv) Tossing a fair coin has a probability of ______ for getting heads.v
Answer:

(i) $0$
(ii) sample space
(iii) $1$
(iv) $\dfrac{1}{2}$

Q.2In a survey of 50 students, 15 students said they liked football. The number of students who like football is 15, and the ________ (frequency/relative frequency) is __________ (fill in the fraction or decimal).v
Solution

Frequency is the count of times an outcome occurs. Relative frequency is the count divided by the total number of observations.

Answer:

The number 15 is the frequency, and the relative frequency is $\dfrac{15}{50}=\dfrac{3}{10}=0.3$.

Q.3Which of the following experiments have equally likely outcomes? Explain. (i) A driver attempts to start a car. The car starts or does not start. (ii) Tossing a fair coin once. (iii) Rolling a fair 6-sided die. (iv) Choosing a marble randomly from a bag that contains 3 red marbles and 7 blue marbles. (v) A baby is born. It is a boy or a girl.v
Solution

Equally likely outcomes require symmetry or equal counts with no bias. A fair coin has H and T equally likely, and a fair die has all six faces equally likely. A car starting depends on its condition. A bag with 3 red and 7 blue marbles gives unequal colour probabilities. Birth outcomes may be close, but they are not assumed exactly equal in the same theoretical way as a fair coin.

Answer:

(i) Not equally likely.
(ii) Equally likely.
(iii) Equally likely.
(iv) Not equally likely if outcomes are recorded by colour; blue is more likely than red.
(v) Not treated as exactly equally likely without data; it is not a designed fair random device like a coin or die.

Q.4Write the sample space and calculate the probability based on the given information. (i) Two coins are tossed at the same time. What is the probability of getting at least one head? (ii) Ten identical cards numbered 1 to 10 are placed in a box. One card is drawn at random. What is the probability of drawing a card with an even number? (iii) A die is rolled once. What is the probability of getting a number greater than 4? (iv) A bag contains 3 red balls, 2 blue balls, and 1 green ball. One ball is picked at random. What is the probability that it is not red? (v) Three coins are tossed simultaneously. What is the probability of getting exactly two heads?v
Solution

Probability is favourable outcomes divided by total equally likely outcomes. For two coins, at least one head means HH, HT or TH. For cards 1 to 10, even cards are 2,4,6,8,10. For one die, numbers greater than 4 are 5 and 6. For the balls, not red means blue or green, $2+1=3$ balls out of 6. For three coins, exactly two heads are HHT, HTH and THH.

Answer:

(i) $S=\{HH,HT,TH,TT\}$; probability $=\dfrac{3}{4}$.
(ii) $S=\{1,2,3,4,5,6,7,8,9,10\}$; probability $=\dfrac{5}{10}=\dfrac12$.
(iii) $S=\{1,2,3,4,5,6\}$; probability $=\dfrac{2}{6}=\dfrac13$.
(iv) By individual balls, total outcomes $=6$; not red outcomes $=3$; probability $=\dfrac12$.
(v) $S=\{HHH,HHT,HTH,THH,HTT,THT,TTH,TTT\}$; probability $=\dfrac{3}{8}$.

Q.5A bag has 3 candies: strawberry, lemon, and mint. One is picked at random. What is the probability of picking a strawberry candy?v
Solution

There are 3 equally likely candies and 1 favourable outcome, strawberry. Therefore the probability is $1/3$.

Answer:

$\dfrac{1}{3}$.

Q.6A child has 2 shirts (one red and one blue) and 3 types of pants (jeans, khakis, and shorts). List all the possible combinations of outfits consisting of one shirt and one pair of pants. Display your answer in a table format.v
Solution

Each of the 2 shirts can be paired with each of the 3 pants, so there are $2\times3=6$ combinations. A table would have rows Red and Blue, and columns Jeans, Khakis and Shorts.

Answer:

The 6 outfits are: red shirt with jeans, red shirt with khakis, red shirt with shorts, blue shirt with jeans, blue shirt with khakis, and blue shirt with shorts.