NCERT solutions class x
Probability
Exercise 15.1
Five cards, the ten, jack, queen, king and ace of diamonds, are well-shuffled
with their face downwards. One card is then picked up at random.
(i) What is the probability that the card is the queen?
(ii) If the queen is drawn and put aside, what is the probability that the
second card picked up is
(a) an ace?
(b) a queen?
Answer
Total number of cards = 5
(i) Number of queens = 1
P (picking a queen) = 1/5
= 0.2
(ii) If the queen is drawn and put aside, the total number of cards left is (5-4) = 4
(a) Total numbers of ace = 1
P (picking an ace) = ¼ = 0.25
(b) Total number of queens = 0
P (picking a queen) = 0/4 = 0
Question 16
12 defective pens are accidentally mixed with 132 good ones. It is not
possible to just look at a pen and tell whether or not it is defective. One pen
is taken out at random from this lot. Determine the probability that the pen
taken out is a good one.
Answer
Numbers of pens = Numbers of defective pens + Numbers of good pens
∴ Total number of pens = 132+12
= 144 pens
P(picking a good pen) = No .of favorable outcomes
Total number of outcomes
= 132/144
= 11/12
= 0.916
Question 17
(i) A lot of 20 bulbs contain 4 defective ones. One bulb is drawn at random
from the lot. What is the probability that this bulb is defective?
(ii) Suppose the bulb drawn in (i) is not defective and is not replaced. Now
one bulb is drawn at random from the rest. What is the probability that this
bulb is not defective?
Answer
(i) Number of defective bulbs = 4
The total number of bulbs = 20
P(getting a defective bulb) =No. of favorable outcome
Total number of outcomes
= 4/20
= 1/5= 0.2
(ii) Since 1 non-defective bulb is drawn, then the total number of bulbs left is 19
So, the total number of events (or outcomes) = 19
No. of non-defective bulbs = 19-4 = 15
Therefore, the probability that the bulb is not defective = 15/19 = 0.789
Question 18
A box contains 90 discs which are numbered from 1 to 90. If one disc
is drawn at random from the box, find the probability that it bears
(i) a two-digit number
(ii) a perfect square number
(iii) a number divisible by 5
Answer
The total number of discs = 90
(i) No. of discs having two digit numbers = 81
Since 1 to 9 are single-digit numbers and
so, total 2-digit numbers are 90-9 = 81
P (bearing a two-digit number) = No. of favorable outcomes
Total no. of outcomes
=81/90
= 9/10 = 0.9
(ii) No of perfect square numbers = 9 (1, 4, 9, 16, 25, 36, 49, 64 and 81)
P (getting a perfect square number) = No. of favorable outcomes
Total no. of outcomes
=9/90
= 1/10 = 0.1
(iii) Numbers which are divisible by 5 = 18
(5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85 and 90)
P (getting a number divisible by 5) = No. of favorable outcomes
Total no . of outcomes
=18/90
= 1/5 = 0.2
Question 19
A child has a die whose six faces show the letters as given below:
The die is thrown once. What is the probability of getting
(i) A?
(ii) D?
Answer
The total number of possible outcomes (or events) = 6
(i) No. of faces having A on it = 2
P (getting A) = No. of favorable outcomes
Total no. of outcomes
=2/6
=1/3
= 0.33
(ii) The total number of faces having D on it = 1
P (getting D) =No. of favorable outcomes
Total no. of outcomes
=1/ 6
= 0.166
Question 20
Suppose you drop a die at random on the rectangular region shown in Fig. 15.6. What is the probability that it will land inside the circle with a diameter 1m?
Answer
First, calculate the area of the rectangle and the area of the circle
Area of the rectangle = (3×2) m2
= 6 m2
Area of the circle = πr2
= π(½)2 m2
= π/4 m2 = 0.78
∴ The probability that die will land inside the circle = [(π/4)/6]
= π/24
= 0.78/6 = 0.13
Question 21
A lot consists of 144 ball pens, of which 20 are defective, and the others are
good. Nuri will buy a pen if it is good, but will not buy it if it is defective. The
shopkeeper draws one pen at random and gives it to her. What is the
probability that
(i) She will buy it?
(ii) She will not buy it?
Answer
The total number of pens = 144
No. of defective pens = 20
∴ The No. of non defective pens = 144-20 = 124
(i) No .of of events in which she will buy them = 124
P (buying) = No .of favorable outcomes
Total no. of outcomes
=124/144
= 31/36 = 0.86
(ii) Number of events in which she will not buy them = 20
P (not buying) = No. of favorable outcomes
Total no. of outcomes
=20/144
= 5/36 = 0.138
Question 22
Refer to Example 13. (i) Complete the following table:
(ii) A student argues that ‘there are 11 possible outcomes 2, 3, 4, 5, 6, 7, 8, 9,
10, 11 and 12. Therefore, each of them has a probability of 1/11. Do you agree
with this argument? Justify your Solution:
Answer
If 2 dice are thrown, the possible events are:
(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)
Total number of events: 6×6 = 36
(i) It is given that to get the sum as 2, the only possible outcomes = (1,1)
p(getting sum 2) = No. of favorable outcomes
Total no. of outcomes
= 1/36
Getting the sum as 3, the possible events (or outcomes) = (1,2) and (2,1)
P( Getting sum 3) =No. of favorable outcomes
Total no. of outcomes
= 2/36
Similarly,
Getting the sum as 4= (1,3), (3,1), and (2,2)
P (getting sum 4) =No. of favorable outcomes
Total no. of outcomes
=3/36
Getting sum as 5 =(1,4), (4,1), (2,3), and (3,2)
P (getting the sum 5) = No. of favorable outcomes
Total no. of outcomes
=4/36
Getting sum as 6 = (1,5), (5,1), (2,4), (4,2), and (3,3)
P (getting the sum 6) = No. of favorable outcomes
Total no. of outcomes
= 5/36
Getting the sum as 7 = (1,6), (6,1), (5,2), (2,5), (4,3), and (3,4)
P (getting the sum 7) = No. of favorable outcomes
Total no. of outcomes
= 6/36
Getting the sum as 8 = (2,6), (6,2), (3,5), (5,3), and (4,4)
P ( getting the sum 8) =No. of favorable outcomes
Total no. of outcomes
= 5/36
Getting the sum as 8 = (3,6), (6,3), (4,5), and (5,4)
P (getting the sum 9) =No. of favorable outcomes
Total no. of outcomes
=4/36
Getting the sum as 10 = (4,6), (6,4), and (5,5)
P ( getting the sum 10) = No. of favorable outcomes
Total no. of outcomes
= 3/36
Getting the sum as 11= (5,6), and (6,5)
P ( getting the sum 11) = No. of favorable outcomes
Total no. of outcomes
= 2/36
Getting the sum as 12= (6,6)
P ( getting the sum 12) =No. of favorable outcomes
Total no. of outcomes
= 1/36
So, the table will be as:
Event: Sum on 2 dice | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
Probability | 1/36 | 2/36 | 3/36 | 4/36 | 5/36 | 6/36 | 5/36 | 4/36 | 3/36 | 2/36 | 1/36 |
(ii) The argument is not correct as it is already justified in (i) that the number of
all possible outcomes is 36 and not 11.
Question 23
A game consists of tossing a one-rupee coin 3 times and noting its outcome
each time. Hanif wins if all the tosses give the same result, i.e. three heads or
three tails, and loses otherwise. Calculate the probability that Hanif will lose
the game.
Answer
The total number of outcomes = 8 (HHH, HHT, HTH, THH, TTH, HTT, THT, TTT)
Total outcomes in which Hanif will lose the game = 6 (HHT, HTH, THH, TTH,
HTT, THT)
P (losing the game) = No. of favorable outcomes
Total no. of outcomes
= 6/8
=3/4 = 0.75
Question 24
A die is thrown twice. What is the probability that
(i) 5 will not come up either time?
(ii) 5 will come up at least once?
[Hint: Throwing a die twice and throwing two dice simultaneously are treated as the
same experiment]
Answer
Outcomes are:
(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)
So, the total number of outcomes = 6×6 = 36
(i)Total no. of outcomes when 5 comes up on either time are; (5,1),(5,2),(5,3),(5,4)
(5,5),(5,6)
Hence total no. of favourable cases =11
P(5 will come up either time) =11/36
P(5 will not come up either time) =1 -11/36
=25/36
(ii) Number of events when 5 comes at least once = 11
∴ The required probability = 11/36
Question 25
Which of the following arguments are correct and which are not correct?
Give reasons for your solution:
(i) If two coins are tossed simultaneously, there are three possible outcomes
– two heads, two tails or one of each. Therefore, for each of these outcomes,
the probability is 1/3
(ii) If a die is thrown, there are two possible outcomes – an odd number or an
even number. Therefore, the probability of getting an odd number is 1/2
Answer
(i) All the possible events are (H,H); (H,T); (T,H) and (T,T)
P (getting two heads) = 1/4
P (getting one of each) = 2/4
=1/2
∴ This statement is incorrect.
(ii) Since the two outcomes are equally likely, this statement is correct.
Exercise: 15.2 (Page No: 311)
Question 1
Two customers, Shyam and Ekta, are visiting a particular shop in the same
week (Tuesday to Saturday). Each is equally likely to visit the shop on any
day as on another day. What is the probability that both will visit the shop on
(i) the same day?
(ii) consecutive days?
(iii) different days?
Answer
Since there are 5 days and both can go to the shop in 5 ways each so,
The total number of possible outcomes = 5×5 = 25
(i) The number of favourable events = 5 (Tue., Tue.), (Wed., Wed.), (Thu.,
Thu.), (Fri., Fri.), (Sat., Sat.)
P (both visiting on the same day) = No. of favorable outcomes
Total no. of outcomes
=5/25
= 1/5
(ii) The number of favourable events = 8 (Tue., Wed.), (Wed., Thu.), (Thu.,
Fri.), (Fri., Sat.), (Sat., Fri.), (Fri., Thu.), (Thu., Wed.), and (Wed., Tue.)
P(both visiting on the consecutive days) = No. of favorable outcomes
Total no. of outcomes
= 8/25
(iii) P (both visiting on different days) = 1-P (both visiting on the same day)
P (both visiting on different days) = 1-(⅕)
=4/5
Question 2
A die is numbered in such a way that its faces show the numbers 1, 2, 2, 3, 3,
6. It is thrown two times, and the total score in two throws is noted. Complete
the following table, which gives a few values of the total score on the two
throws:
What is the probability that the total score is
(i) even?
(ii) 6?
(iii) at least 6?
Answer
The table will be as follows:
+ | 1 | 2 | 2 | 3 | 3 | 6 |
1 | 2 | 3 | 3 | 4 | 4 | 7 |
2 | 3 | 4 | 4 | 5 | 5 | 8 |
2 | 3 | 4 | 4 | 5 | 5 | 8 |
3 | 4 | 5 | 5 | 6 | 6 | 9 |
3 | 4 | 5 | 5 | 6 | 6 | 9 |
6 | 7 | 8 | 8 | 9 | 9 | 12 |
So, the total number of outcomes = 6×6 = 36
(i) Getting an even = 18
P (Even) = No. of favorable outcomes
Total no. of outcomes
=18/36
= ½
(ii) Getting the sum 6= 4
P (sum is 6) = No. of favorable outcomes
Total no. of outcomes
= 4/36 = 1/9
(iii) Getting the sum is atleast 6 = 15
P (sum is atleast 6) = No. of favorable outcomes
Total no. of outcomes
=15/36 = 5/12
Question 3
A bag contains 5 red balls and some blue balls. If the probability of drawing
a blue ball is double that of a red ball, determine the number of blue balls in
the bag.
Answer
It is given that the total number of red balls = 5
Let the total number of blue balls = x
∴ P (drawing a blue ball) =No. of favorable outcomes
Total no. of outcomes
= [x/(x+5)] ——–(i)
Similarly,
P (drawing a red ball) = No. of favorable outcomes
Total no. of outcomes
= [5/(x+5)] ——–(i)
From equations (i) and (ii)
x = 10
So, the total number of blue balls = 10
Question 4
A box contains 12 balls, out of which x are black. If one ball is drawn at
random from the box, what is the probability that it will be a black ball?
If 6 more black balls are put in the box, the probability of drawing a black ball
is now double of what it was before. Find x
Answer
Number of black balls = x
Total number of balls = 12
P (getting black balls) = No. of favorable outcomes
Total no. of outcomes
= x/12 ——————-(i)
Now, when 6 more black balls are added,
Total balls become = 18
∴ Total number of black balls = x+6
Now, P (getting black balls) =No. of favorable outcomes
Total no. of outcomes
= (x+6)/18 ——————-(ii)
It’s given that the probability of drawing a black ball now is double of
what it was before.
(x+6)/18 = 2 × (x/12)
x + 6 = 3x
2x = 6
∴ x = 3
Question 5
A jar contains 24 marbles, some are green, and others are blue. If a marble is
drawn at random from the jar, the probability that it is green is ⅔. Find the
number of blue balls in the jar.
Answer
Total marbles = 24
Let the green marbles = x
So, the blue marbles = 24-x
P(getting green marble) = No. of favorable outcomes
Total no. of outcomes
= x/24
From the question, x/24 = 2/3
So, the total green marbles = 16
And, the total blue marbles = 24-16 = 8
Introduction
Probability is a branch of mathematics that deals with the likelihood or chance of events occurring. In the Central Board of Secondary Education (CBSE) Class 10 curriculum, probability is introduced as an essential topic in the field of statistics. This article aims to provide an overview of probability for Class 10 CBSE students, covering key concepts, methods of calculation, and practical applications.
Understanding Probability
Sample Space: In probability, a sample space refers to the set of all possible outcomes of an experiment. It is denoted by the symbol "S." For example, when tossing a fair coin, the sample space consists of two outcomes: heads (H) and tails (T). Understanding the sample space is crucial for calculating probabilities accurately.
Events:
Probability of an Event:
Methods of Calculating Probability
Theoretical Probability:
Experimental Probability:
Addition Rule of Probability:
Multiplication Rule of Probability: The multiplication rule is used to calculate the probability of the intersection of two or more events. If E and F are two events, the probability of both E and F occurring is given by P(E and F) = P(E) × P(F|E), where P(F|E) represents the probability of F given that E has already occurred.
Practical Applications of Probability
Games of Chance:
Risk Assessment:
Weather Forecasting:
Opinion Polls and Surveys:
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