EXERCISE 15.1
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Question 1:
In a cricket math, a batswoman hits a boundary 6 times out of 30 balls she plays. Find the probability that she did not hit a boundary.
Answer:
Number of times the batswoman hits a boundary = 6
Total number of balls played = 30
∴ Number of times that the batswoman does not hit a boundary = 30 − 6 = 24
Question 2:
1500 families with 2 children were selected randomly, and the following data were recorded:
Number of girls in a family 
2 
1 
0 
Number of families 
475 
814 
211 
Compute the probability of a family, chosen at random, having
(i) 2 girls (ii) 1 girl (iii) No girl
Also check whether the sum of these probabilities is 1.
Answer:
Total number of families = 475 + 814 + 211
= 1500
(i) Number of families having 2 girls = 475
(ii) Number of families having 1 girl = 814
(iii) Number of families having no girl = 211
Therefore, the sum of all these probabilities is 1.
Question 3:
In a particular section of Class IX, 40 students were asked about the months of their birth and the following graph was prepared for the data so obtained:
Find the probability that a student of the class was born in August.
Answer:
Number of students born in the month of August = 6
Total number of students = 40
Question 4:
Three coins are tossed simultaneously 200 times with the following frequencies of different outcomes:

Outcome
3 heads
2 heads
1 head
No head
Frequency
23
72
77
28
If the three coins are simultaneously tossed again, compute the probability of 2 heads coming up
Answer:
Number of times 2 heads come up = 72
Total number of times the coins were tossed = 200
Question 5:
An organization selected 2400 families at random and surveyed them to determine a relationship between income level and the number of vehicles in a family. The information gathered is listed in the table below:
Suppose a family is chosen, find the probability that the family chosen is
(i) earning Rs 10000 − 13000 per month and owning exactly 2 vehicles.
(ii) earning Rs 16000 or more per month and owning exactly 1 vehicle.
(iii) earning less than Rs 7000 per month and does not own any vehicle.
(iv) earning Rs 13000 − 16000 per month and owning more than 2 vehicles.
(v) owning not more than 1 vehicle.
Answer:
Number of total families surveyed = 10 + 160 + 25 + 0 + 0 + 305 + 27 + 2 + 1 + 535 + 29 + 1 + 2 + 469 + 59 + 25 + 1 + 579 + 82 + 88 = 2400
(i) Number of families earning Rs 10000 − 13000 per month and owning exactly 2 vehicles = 29
Hence, required probability,
(ii) Number of families earning Rs 16000 or more per month and owning exactly 1 vehicle = 579
Hence, required probability,
(iii) Number of families earning less than Rs 7000 per month and does not own any vehicle = 10
Hence, required probability,
(iv) Number of families earning Rs 13000 − 16000 per month and owning more than 2 vehicles = 25
Hence, required probability,
(v) Number of families owning not more than 1 vehicle = 10 + 160 + 0 + 305 + 1 + 535 + 2 + 469 + 1 + 579 = 2062
Hence, required probability,
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Question 6:
A teacher wanted to analyse the performance of two sections of students in a mathematics test of 100 marks. Looking at their performances, she found that a few students got under 20 marks and a few got 70 marks or above. So she decided to group them into intervals of varying sizes as follows: 0 − 20, 20 − 30… 60 − 70, 70 − 100. Then she formed the following table:
Marks 
Number of student 
0 − 20 20 − 30 30 − 40 40 − 50 50 − 60 60 − 70 70 − above 
7 10 10 20 20 15 8 
Total 
90 
(i) Find the probability that a student obtained less than 20 % in the mathematics test.
(ii) Find the probability that a student obtained marks 60 or above.
Answer:
Totalnumber of students = 90
(i) Number of students getting less than 20 % marks in the test = 7
Hence, required probability,
(ii) Number of students obtaining marks 60 or above = 15 + 8 = 23
Hence, required probability,
Question 7:
To know the opinion of the students about the subject statistics, a survey of 200 students was conducted. The data is recorded in the following table.

Opinion
Number of students
like
dislike
135
65
Find the probability that a student chosen at random
(i) likes statistics, (ii) does not like it
Answer:
Total number of students = 135 + 65 = 200
(i) Number of students liking statistics = 135
(ii) Number of students who do not like statistics = 65
Question 8:
The distance (in km) of 40 engineers from their residence to their place of work were found as follows.
5  3  10  20  25  11  13  7  12  31 
19  10  12  17  18  11  32  17  16  2 
7  9  7  8  3  5  12  15  18  3 
12  14  2  9  6  15  15  7  6  12 
What is the empirical probability that an engineer lives:
(i) less than 7 km from her place of work?
(ii) more than or equal to 7 km from her place of work?
(iii) within km from her place of work?
Answer:
(i) Total number of engineers = 40
Number of engineers living less than 7 km from their place of work = 9
Hence, required probability that an engineer lives less than 7 km from her place of work,
(ii) Number of engineers living more than or equal to 7 km from their place of work = 40 − 9 = 31
Hence, required probability that an engineer lives more than or equal to 7 km from her place of work,
(iii) Number of engineers living within km from her place of work = 0
Hence, required probability that an engineer lives within km from her place of work, P = 0
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Question 9:
Note the frequency of twowheelers, threewheelers and fourwheelers going past during a time interval, in front of your school gate. Find the probability that any one vehicle out of the total vehicles you have observed is a twowheeler.
Answer:
This is an activity based question. Students are advised to perform this activity by yourself.
Question 10:
Ask all the students in your class to write a 3digit number. Choose any student from the room at random. What is the probability that the number written by her/him is divisible by 3? Remember that a number is divisible by 3, if the sum of its digits is divisible by 3.
Answer:
This is an activity based question. Students are advised to perform this activity by yourself.
Question 11:
Eleven bags of wheat flour, each marked 5 kg, actually contained the following weights of flour (in kg):
4.97 5.05 5.08 5.03 5.00 5.06 5.08 4.98 5.04 5.07 5.00
Find the probability that any of these bags chosen at random contains more than 5 kg of flour.
Answer:
Number of total bags = 11
Number of bags containing more than 5 kg of flour = 7
Hence, required probability,
Question 12:
Concentration of SO_{2} (in ppm) 
Number of days (frequency ) 
0.00 − 0.04 
4 
0.04 − 0.08 
9 
0.08 − 0.12 
9 
0.12 − 0.16 
2 
0.16 − 0.20 
4 
0.20 − 0.24 
2 
Total 
30 
The above frequency distribution table represents the concentration of sulphur dioxide in the air in parts per million of a certain city for 30 days. Using this table, find the probability of the concentration of sulphur dioxide in the interval 0.12 − 0.16 on any of these days.
Answer:
Number days for which the concentration of sulphur dioxide was in the interval of 0.12 − 0.16 = 2
Total number of days = 30
Hence, required probability,
Question 13:
Blood group 
Number of students 
A 
9 
B 
6 
AB 
3 
O 
12 
Total 
30 
The above frequency distribution table represents the blood groups of 30 students of a class. Use this table to determine the probability that a student of this class, selected at random, has blood group AB.
Answer:
Number of students having blood group AB = 3
Total number of students = 30
Hence, required probability,