#### Chapter 24 Measure of Central TendencyÂ R.D. Sharma Solutions for Class 9th Exercise 24.1

**Exercise 24.1**

1.Â If the height of 5 persons are 140 cm, 150 cm, 152 cm, 158 cm and 161 cm respectively, find the mean height.

**Solution**

Given,

The heights of 5 persons are 140 cm , 150 cm , 152 cm , 158 cm and 161 cm

âˆ´ Mean Weight = sum of heights / total no. of persons

= 140+150+152+158+161/5

= 761/5 = 152.2

2. Find the mean of 994, 996, 998, 1002, 1000.

**Solution**

Given,

Numbers are 994,996,998,1000,1002

âˆ´ Mean Weight = sum of heights/total no. of persons

= 994+996+998+1000+1002/5

= 4990/5 = 998

Mean = 998

3. Find the mean of first five natural numbers.

**Solution**

The first five odd numbers are 1 , 2 , 3 , 4 , 5.

âˆ´ Mean = sum of numbers/total numbers

= 1+2+3+4+5/5

= 15/5 = 3

Mean = 3

4. Find the mean of all factors of 10.

**Solution**

All factors of 6 are 1 , 2 , 5 , 10.

âˆ´ Mean = sum of factors/ total factors

= 1+2+5+10/4 = 4.5

Mean = 4.5

5. Find the mean of first 10 even natural numbers.

**Solution**

The first five even natural numbers are 2 , 4 , 6 , 8 , 10 , 12 , 14 , 16 , 18 , 20

âˆ´ Mean = sum of numbers / total numbers

= 2+4+6+8+10+12+14+16+18+20/10 = 11

Mean = 11

6.Â Find the mean of x, x+2, x+4, x+6, x+8.

**Solution**

Numbers are x , x + 2 , x + 4 , x + 6 , x + 8.

âˆ´ Mean = sum of numbers total numbers

= x + x + 2 + x + 4 + x + 6 + x + 85

= 5x + 20/5

= 5(x+4/5)

= x + 4

7. Find the mean of first five multiples of 3.

**Solution**

First five multiples of 3 are 3 , 6 , 9 , 12 , 15.

âˆ´ Mean = sum of numbers/total numbers

= 3+6+9+12+15/5

= 9

Mean = 9

8. Following are the weights of 10 new born babies in a hospital on a particular day : 3.4 , 3 .6 , 4.2 , 4.5 , 3.9 , 4.1 , 3.8 , 4.5 , 4.4 , 3.6 (in kg). Find the mean.

**Solution**

The weights (in kg) of 10 new bornÂ babies are : 3.4 , 3 .6 , 4.2 , 4.5 , 3.9 , 4.1 , 3.8 , 4.5 , 4.4 , 3.6

âˆ´Â Mean Weight = sum of weights/total no.of babies

= 3.4+3.6+4.2+4.5+3.9+4.1+3.8+4.5+4.4+3.610

= 4 kg

9. The percentage marks obtained by students of a class in mathematics are as follows: 64 , 36 , 47 , 23 , 0 , 19 , 81 , 93 , 72 , 35 , 3 , 1 . Find their mean.

**Solution**

The percentage marks obtained by students are 64 , 36 , 47 , 23 , 0 , 19 , 81 , 93 , 72 , 35 , 3 , 1

âˆ´ Mean marks = sum of marks/total numbers of marks

= 64+36+47+23+0+19+81+93+72+35+3+1/5 =39.5

Mean Marks = 39.5

10. The numbers of children in 10 families of a locality are 2 , 4 , 3 , 4 , 2 , 3 , 5 , 1 , 1 , 5 . Find the mean number of children per family.

**Solution**

The numbers of childrenÂ in 10 familiesÂ are : 2 , 4 , 3 , 4 , 2 , 3 , 5 , 1 , 1 , 5

âˆ´ Mean = total no.children/total families

= 2+4+3+4+2+3+5+1+1+5/10 = 3

11. If M is the mean of x

_{1}, x

_{2}, x

_{3}, x

_{5}Â and x

_{6}, prove that

(x

_{1}Â âˆ’ M) + (x

_{2}Â âˆ’ M) (x

_{3}Â âˆ’ M) (x

_{4}Â âˆ’ M) + (x

_{5}Â âˆ’ M) + (x

_{6}Â âˆ’ M) = 0.

**Solution**Â

12. Durations of sunshine (in hours) in Amritsar to first 10 days of August 1997 as reported by the Meteorological Department are given below:

9.6, 5.2, 3.5, 1.5, 1.6, 2.4, 2.6, 8.4, 10.3, 10.9

(i) Find the mean xÌ… .

**SolutionÂ**

Given that the duration of sunshine (in hours) in Amritsar for 10 days are 9.6, 5.2, 3.5, 1.5, 1.6, 2.4, 2.6, 8.4, 10.3, and 10.9.

(i) We have to find their mean.

Remember the definition of mean of n values x

_{1}, x

_{2}â€¦ x

_{n}Â is

13. Explain, by taking a suitable example, how the arithmetic mean alters by (i) adding a constant k to each term, (ii) subtracting a constant k from each them, (iii) multiplying each term by a constant k and (iv) dividing each term by a non-zero constant k.

**Solution**

Let say numbers are 3 , 4 , 5

âˆ´ Mean = sum of numbers total numbers

= 3+4+5/3 = 4 .

(i) Adding constant term k = 2 in each term.

New numbers are = 5 , 6 , 7

âˆ´ Mean = sum of numbers total numbers

= 5+6+7/3

= 6 = 4 + 2

âˆ´ newÂ mean will be 2 more than the original mean.

New numbers are = 1 , 2 , 3

âˆ´ MeanÂ Â Â = sum of numbers total numbers

= 1+2+3/3

= 2 = 4 â€“ 2

New numbers are = 6 , 8 , 10

âˆ´ MeanÂ Â Â = sum of numbers total numbers

= 6+8+10/3

= 8 = 4Ã—2

âˆ´ newÂ mean will be 2 times ofÂ the original mean.

(iv) Divide the constant term k =2 in each term.

New numbers are = 1.5 , 2 , 2.5.

âˆ´ MeanÂ Â Â = sum of numbers/total numbers

= 1.5+2+2.53

= 2 = 42

âˆ´ newÂ mean will be halfÂ ofÂ the original mean.

**Solution**

Mean marks of 100 students = 40

Sum of marks of 100 students = 100Ã—40

= 4000

Correct value = 53

Incorrect value = 83

Correct sum = 4000 â€“ 83 + 53 = 3970

âˆ´ correct mean = 3970/100 = 39.7

15. The traffic police recorded the speed (in km/hr) of 10 motorists as 47 , 53 , 49 , 60 , 39 , 42 , 55 , 57 , 52 , 48 . Later on, an error in recording instrument was found. Find the correct average speed of the motorists if the instrument is recorded 5 km/hr less in each case.

**Solution**

The speed of 10 motorists are 47 , 53 , 49 , 60 , 39 , 42 , 55 , 57 , 52 , 48 .

Later on it was discovered that the instrument recorded 5 km/hr less than in each case

âˆ´ correct values are = 52 , 58 , 54 , 65 , 44 , 47 , 60 , 62 , 57 , 53.

âˆ´ correct mean = 52+58+54+65+44+47+60+62+57+53/10

= 552/10 =55.2 km/hr .

16. The mean of five numbers is 27. If one number is excluded, their mean is 25. Find the excluded number.

**Solution**

The mean of five numbers is 27

The sum of five numbers = 5Ã—27 = 135

If one number is excluded , the new mean is 25

âˆ´ Sum of 4 numbers = 4Ã—25 = 100

âˆ´ Excluded number = 135 â€“ 100 = 35

17. The mean weight per student in a group of 7 students is 55 kg. The individual weights of 6 of them (in kg) are 52, 54, 55, 53, 56 and 54. Find the weight of the seventh student.

**Solution**

The mean weight per student in a group of 7 students = 55 kg

Weight of 6 students (in kg) = 52 , 54 , 55 , 53 , 56 and 54

Let the weight of seventh student = x kg

âˆ´ Mean Weight = sum of weights/total no.of students

â‡’ 55 = (52+54+55+53+56+54+x)/ 7

â‡’ 385 = 324 + x â‡’ x = 385âˆ’324

â‡’ x = 61 kg

âˆ´ weight of seventh student = 61 kg.

18. The mean weight of 8 numbers is 15. If each number is multiplied by 2 , what will be the new mean ?

**Solution**

We have ,

The mean weight of 8 numbers is 15

Then , the sum of 8 numbers = 8Ã—15 = 120

If each number is multiplied by 2

Then , new mean = 120Ã—2 = 240

âˆ´ NewÂ mean = 240/8 = 30.

19. The mean of 5 numbers is 18. If one number is excluded, their mean is 16. Find the excluded number.

**Solution**

The mean of 5 numbers is 18

Then , the sum of 5 numbers = 5Ã—18 = 90

If one number is excluded

Then , the mean of 4 numbers = 16

âˆ´ sum of 4 numbers = 4Ã—16 = 64

Excluded number = 90 â€“ 64 = 26.

20. The mean of 200 items was 50. Later on, it was on discovered that the two items were misread as 92 and 8 instead of 192 and 88. Find the correct mean.

**Solution**

The mean of 200 items = 50

Then the sum of 200 items = 200Ã—50 = 10,000

Correct values = 192 and 88.

Incorrect values = 92 and 8.

âˆ´ correct sum = 10000 - 92 - 8 + 192 + 88 = 10180

âˆ´ correct mean = 10180/200 = 101.8/2 = 50.9 .

21. Find the values of n and X in each of the following cases:

**Solution**

_{1}, x

_{2},...., x

_{n}, measured from 15 and -3 are -90 and 54 respectively. Find the value of n and mean.

**Solution**

23. Find the sum of the deviations of the variate values 3, 4, 6, 7, 8, 14 from their mean.

**Solution**

24. If xÌ… is the mean of the ten natural numbersÂ x

_{1},Â x_{1},Â x_{1}Â ....,x_{1}, Show that (x_{1}Â -Â xÌ…) + (x_{2}Â -Â xÌ…) + ... + (x_{10}Â -Â xÌ…) = 0.**Â Solution**