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NCERT solution class 10 chapter 15 Statistics exercise 15.3 mathematics

EXERCISE 15.3


Page No 287:

Question 1:

The following frequency distribution gives the monthly consumption of electricity of 68 consumers of a locality. Find the median, mean and mode of the data and compare them.

Monthly consumption (in units)

Number of consumers

65 − 85

4

85 − 105

5

105 − 125

13

125 − 145

20

145 − 165

14

165 − 185

8

185 − 205

4

Answer:

To find the class marks, the following relation is used.

Taking 135 as assumed mean (a), diui, fiui are calculated according to step deviation method as follows.

Monthly consumption (in units)

Number of consumers (i)

xi class mark

dixi− 135

65 − 85

4

75

− 60

− 3

− 12

85 − 105

5

95

− 40

− 2

− 10

105 − 125

13

115

− 20

− 1

− 13

125 − 145

20

135

0

0

0

145 − 165

14

155

20

1

14

165 − 185

8

175

40

2

16

185 − 205

4

195

60

3

12

Total

68

7

From the table, we obtain

From the table, it can be observed that the maximum class frequency is 20, belonging to class interval 125 − 145.

Modal class = 125 − 145

Lower limit (l) of modal class = 125

Class size (h) = 20

Frequency (f1) of modal class = 20

Frequency (f0) of class preceding modal class = 13

Frequency (f2) of class succeeding the modal class = 14

To find the median of the given data, cumulative frequency is calculated as follows.

Monthly consumption (in units)

Number of consumers

Cumulative frequency

65 − 85

4

4

85 − 105

5

4 + 5 = 9

105 − 125

13

9 + 13 = 22

125 − 145

20

22 + 20 = 42

145 − 165

14

42 + 14 = 56

165 − 185

8

56 + 8 = 64

185 − 205

4

64 + 4 = 68

From the table, we obtain

n = 68

Cumulative frequency (cf) just greater than is 42, belonging to interval 125 − 145.

Therefore, median class = 125 − 145

Lower limit (l) of median class = 125

Class size (h) = 20

Frequency (f) of median class = 20

Cumulative frequency (cf) of class preceding median class = 22

Therefore, median, mode, mean of the given data is 137, 135.76, and 137.05 respectively.

The three measures are approximately the same in this case.

Video Solution

Question 2:

If the median of the distribution is given below is 28.5, find the values of x and y.

Class interval

Frequency

0 − 10

5

10 − 20

x

20 − 30

20

30 − 40

15

40 − 50

y

50 − 60

5

Total

60

Answer:

The cumulative frequency for the given data is calculated as follows.

Class interval

Frequency

Cumulative frequency

0 − 10

5

5

10 − 20

x

5+ x

20 − 30

20

25 + x

30 − 40

15

40 + x

40 − 50

y

40+ x + y

50 − 60

5

45 + x + y

Total (n)

60

From the table, it can be observed that = 60

45 + x + y = 60

x + y = 15 (1)

Median of the data is given as 28.5 which lies in interval 20 − 30.

Therefore, median class = 20 − 30

Lower limit (l) of median class = 20

Cumulative frequency (cf) of class preceding the median class = 5 + x

Frequency (f) of median class = 20

Class size (h) = 10

From equation (1),

8 + y = 15

y = 7

Hence, the values of x and y are 8 and 7 respectively.


Question 3:

A life insurance agent found the following data for distribution of ages of 100 policy holders. Calculate the median age, if policies are given only to persons having age 18 years onwards but less than 60 year.

Age (in years)

Number of policy holders

Below 20

2

Below 25

6

Below 30

24

Below 35

45

Below 40

78

Below 45

89

Below 50

92

Below 55

98

Below 60

100

Answer:

Here, class width is not the same. There is no requirement of adjusting the frequencies according to class intervals. The given frequency table is of less than type represented with upper class limits. The policies were given only to persons with age 18 years onwards but less than 60 years. Therefore, class intervals with their respective cumulative frequency can be defined as below.

Age (in years)

Number of policy holders (fi)

Cumulative frequency (cf)

18 − 20

2

2

20 − 25

6 − 2 = 4

6

25 − 30

24 − 6 = 18

24

30 − 35

45 − 24 = 21

45

35 − 40

78 − 45 = 33

78

40 − 45

89 − 78 = 11

89

45 − 50

92 − 89 = 3

92

50 − 55

98 − 92 = 6

98

55 − 60

100 − 98 = 2

100

Total (n)

From the table, it can be observed that n = 100.

Cumulative frequency (cf) just greater than is 78, belonging to interval 35 − 40.

Therefore, median class = 35 − 40

Lower limit (l) of median class = 35

Class size (h) = 5

Frequency (f) of median class = 33

Cumulative frequency (cf) of class preceding median class = 45

Therefore, median age is 35.76 years.

Video Solution

Page No 288:

Question 4:

The lengths of 40 leaves of a plant are measured correct to the nearest millimeter, and the data obtained is represented in the following table:

Length (in mm)

Number or leaves fi

118 − 126

3

127 − 135

5

136 − 144

9

145 − 153

12

154 − 162

5

163 − 171

4

172 − 180

2

Find the median length of the leaves.

(Hint: The data needs to be converted to continuous classes for finding the median, since the formula assumes continuous classes. The classes then change to 117.5 − 126.5, 126.5 − 135.5… 171.5 − 180.5)

Answer:

The given data does not have continuous class intervals. It can be observed that the difference between two class intervals is 1. Therefore,  has to be added and subtracted to upper class limits and lower class limits respectively.

Continuous class intervals with respective cumulative frequencies can be represented as follows.

Length (in mm)

Number or leaves fi

Cumulative frequency

117.5 − 126.5

3

3

126.5 − 135.5

5

3 + 5 = 8

135.5 − 144.5

9

8 + 9 = 17

144.5 − 153.5

12

17 + 12 = 29

153.5 − 162.5

5

29 + 5 = 34

162.5 − 171.5

4

34 + 4 = 38

171.5 − 180.5

2

38 + 2 = 40

From the table, it can be observed that the cumulative frequency just greater than  is 29, belonging to class interval 144.5 − 153.5.

Median class = 144.5 − 153.5

Lower limit (l) of median class = 144.5

Class size (h) = 9

Frequency (f) of median class = 12

Cumulative frequency (cf) of class preceding median class = 17

Median 

Therefore, median length of leaves is 146.75 mm.

Video Solution

Page No 289:

Question 5:

Find the following table gives the distribution of the life time of 400 neon lamps:

Life time (in hours)

Number of lamps

1500 − 2000

14

2000 − 2500

56

2500 − 3000

60

3000 − 3500

86

3500 − 4000

74

4000 − 4500

62

4500 − 5000

48

Find the median life time of a lamp.

Answer:

Thecumulative frequencies with their respective class intervals are as follows.

Life time

Number of lamps (fi)

Cumulative frequency

1500 − 2000

14

14

2000 − 2500

56

14 + 56 = 70

2500 − 3000

60

70 + 60 = 130

3000 − 3500

86

130 + 86 = 216

3500 − 4000

74

216 + 74 = 290

4000 − 4500

62

290 + 62 = 352

4500 − 5000

48

352 + 48 = 400

Total (n)

400

It can be observed that the cumulative frequency just greater than  is 216, belonging to class interval 3000 − 3500.

Median class = 3000 − 3500

Lower limit (l) of median class = 3000

Frequency (f) of median class = 86

Cumulative frequency (cf) of class preceding median class = 130

Class size (h) = 500

Median 

= 3406.976

Therefore, median life time of lamps is 3406.98 hours.

Video Solution

Question 6:

100 surnames were randomly picked up from a local telephone directory and the frequency distribution of the number of letters in the English alphabets in the surnames was obtained as follows:

Number of letters

1 − 4

4 − 7

7 − 10

10 − 13

13 − 16

16 − 19

Number of surnames

6

30

40

6

4

4

Determine the median number of letters in the surnames. Find the mean number of letters in the surnames? Also, find the modal size of the surnames.

Answer:

The cumulative frequencies with their respective class intervals are as follows.

Number of letters

Frequency (fi)

Cumulative frequency

1 − 4

6

6

4 − 7

30

30 + 6 = 36

7 − 10

40

36 + 40 = 76

10 − 13

16

76 + 16 = 92

13 − 16

4

92 + 4 = 96

16 − 19

4

96 + 4 = 100

Total (n)

100

It can be observed that the cumulative frequency just greater than is 76, belonging to class interval 7 − 10.

Median class = 7 − 10

Lower limit (l) of median class = 7

Cumulative frequency (cf) of class preceding median class = 36

Frequency (f) of median class = 40

Class size (h) = 3

Median 

= 8.05

To find the class marks of the given class intervals, the following relation is used.

Taking 11.5 as assumed mean (a), diui, and fiui are calculated according to step deviation method as follows.

Number of letters

Number of surnames

fi

xi

di = xi− 11.5

fiui

1 − 4

6

2.5

− 9

− 3

− 18

4 − 7

30

5.5

− 6

− 2

− 60

7 − 10

40

8.5

− 3

− 1

− 40

10 − 13

16

11.5

0

0

0

13 − 16

4

14.5

3

1

4

16 − 19

4

17.5

6

2

8

Total

100

− 106

From the table, we obtain

fiui = −106

fi = 100

Mean, 

= 11.5 − 3.18 = 8.32

The data in the given table can be written as

Number of letters

Frequency (fi)

1 − 4

6

4 − 7

30

7 − 10

40

10 − 13

16

13 − 16

4

16 − 19

4

Total (n)

100

From the table, it can be observed that the maximum class frequency is 40 belonging to class interval 7 − 10.

Modal class = 7 − 10

Lower limit (l) of modal class = 7

Class size (h) = 3

Frequency (f1) of modal class = 40

Frequency (f0) of class preceding the modal class = 30

Frequency (f2) of class succeeding the modal class = 16

Therefore, median number and mean number of letters in surnames is 8.05 and 8.32 respectively while modal size of surnames is 7.88.

Video Solution

Question 7:

The distribution below gives the weights of 30 students of a class. Find the median weight of the students.

Weight (in kg)

40 − 45

45 − 50

50 − 55

55 − 60

60 − 65

65 − 70

70 − 75

Number of students

2

3

8

6

6

3

2

Answer:

The cumulative frequencies with their respective class intervals are as follows.

Weight (in kg)

Frequency (fi)

Cumulative frequency

40 − 45

2

2

45 − 50

3

2 + 3 = 5

50 − 55

8

5 + 8 = 13

55 − 60

6

13 + 6 = 19

60 − 65

6

19 + 6 = 25

65 − 70

3

25 + 3 = 28

70 − 75

2

28 + 2 = 30

Total (n)

30

Cumulative frequency just greater than  is 19, belonging to class interval 55 − 60.

Median class = 55 − 60

Lower limit (l) of median class = 55

Frequency (f) of median class = 6

Cumulative frequency (cf) of median class = 13

Class size (h) = 5

Median 

= 56.67

Therefore, median weight is 56.67 kg.


 

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