GATEWAY TO CGL MAINS: Concept and Quiz on Average

 We are starting this series from Average on Quant Section .

Tricks and Rules on Average
What is an average?

In simple terms, averages usually refer to the sum of given numbers divided by the total number of terms listed.

Averages = (sum of all terms)/ number of terms

Average is the estimation of the middle number of any series of numbers. 

For example average of 1,2,3,4, 5 is 3.

Average can be calculated by sum of all numbers divided by the total number of numbers

Average of  1,2,3,4,5= (1+2+3+4+5)/5 = 15/5 = 3

Which is also the middle number of the series , from here we can also say that in an A.P. i.e arithmetic progression the middle term is the average of the series .

Rule 1: In the Arithmetic Progression there are two cases when the number of terms is odd and second one is when number of terms is even. So when the number of terms is odd the average will be the middle term. And when the number of terms is even then the average will be the average of two middle terms.

Examples 1: what will be the average of 13, 14, 15, 16, 17?  

Solution: Average is the middle term when the number of terms is odd, but before that let’s checks whether it is in A.P or not, since the common difference is same so the series is in A.P. So the middle term is 15 which is our average of the series. Let’s check it in another way. In the first statement of the article we have written that the average of a set of terms is equal to: Sum of all terms / Number of terms So the sum of all terms in this case is 75 and the number of terms is 5 so the average is 15.

Now come to the second form when the number of terms are even

Rule 2: The average of the series which is in A.P. can be calculated by ½(first + last term) 

Example 1:  What will be the average of 216, 217 , 218?

Solution: So the answer would be = ½ (216 + 218) = 217

(Which is also the middle term of the series)

Rule 3: If the average of n numbers is A and if we add x to each term then the new average will be = (A+ x).

Example 1: The average of 5 numbers is 18. If 4 is added to each of the number then the average would be equal to __?

Solution: Old average = 18

New average will be = 4 + old average = 22

This is because each term is increased by 4 so the average would also be increased by 4 so the new average will be 22

Rule 4: If the average of n numbers is A and if we multiply p with each term then the new average will be = (A x p). For 

Example: The average of 5 numbers is 18. If 4 is multiplied to each of the number then the average would be equal to __?  

Solution: Old average = 18

New average will be = 4 x 18= 72

There are two more operation which can also be applied on the same principle as the above, i.e. subtraction and division.

Rule 5 : In some cases, if a number is included in the series of numbers then the average will change and the value of the newly added term will be = Given average + (number of new terms  x increase in average).This value will also same as the New average + (number of previous terms  x increase in average ) .For 

Example: The average age of 12 students is 40. If the age of the teacher also included then the average becomes 44. Then what will be the age of the teacher?

Solution: Average given = 40

Number of students = 12

Therefore the age of the teacher = 40 + (12 + 1) x 4 = 40 + 52 = 92

And this is also calculated as 44 + (12 x 4)= 92

Therefore the average age of the teacher is 92 yrs

Alternatively 

The average of 12 = 40 that means the total number of units are 12 x 40 = 480

Now the new average is 44 and the number of terms are 13 so therefore the total number of units are = 44 x 13 = 572

So the included units would be equal to 572 – 480 = 92

Rule 6:  In some cases  a number is excluded and one more number is added in the series of the number then the average will change by q and the value of the newly added term will be = Replaced Term + (increased in average x number of terms ).

For Example: The average age of 6 students is increased by 2years when one student whose age was 13 years replaced by a new boy then find the age of the new boy

Solution: The age of the boy will be = Age of the replaced boy +increase in average x number of terms i.e. the age of the newly added boy = 13 + 2 x 6 = 25

Rule 7: There are two more cases when the series is divided into two parts and one of the terms is either included or excluded, then the middle term can be calculated by following methods.

Case 1 : When the term is excluded.

Average(total ) + number of terms in first part x {average (total) – average (first part)} + number of terms in second part x {average (total) – average (second part)}

Case 2: When the term is included.

Average (total) + number of terms in first part x {average (first part) – average(total) }+ x number of terms in second part x {average (second part) – average (total)}

For Example: The average of 20 numbers is 12 .The averages of the first 12 is 11 and the average of next 7 numbers is 10. The last number will be?

Solution: Here in this case one number is excluded so the number would be =

Average(total ) + number of terms in first part x {average (total) – average (first part)} + number of terms in second part x {average (total) – average (second part)} i.e. =  12 + 12 x (12-11)+(12-10) x 7 = 38.

Short Tricks on Average:

Average of the first n natural numbers: (n+1)/2

Average of an arithmetic sequence with first  and last terms known : (1st number+last number)/2

Average of squares of the first n natural numbers: (n+1)(n+2)/6

Average of cubes of the first n natural numbers: n(n+1)^2/4

Average of first n consecutive even number: (n+1)

average of consecutive even number to n : (n+2)/2, where n is the last even number

Average of square n consecutive even number: 2(n+1)(2n+1)/3

Average of square of consecutive even number to n : (n+1)(n+2)/3, where n is the last even number

Average of first n consecutive odd numbers: n

Average of consecutive odd number to n: (n+1)/2

Average of squares of consecutive odd numbers to n: n(n+2)/3, where n is the last odd number
Some Quizes based on above concept and tricks:
1. The average height of the basketball team A is 5 feet 11 inches and that of B is 6 feet 2 inches. There are 20 players in team A and 18 players in team B. The overall average height is:
A. 70.22 inches
B. 70 inches
C. 72.42 inches
D. 72 inches

2.The average age of 14 girls and their teacher's age is 15 years.If the teacher's age excluded, the average reduces by 1. what is the teacher's age?
A. 35 years
B. 32 years
C. 30 years
D. 29 years

3.The average of nine numbers is 50. The average of the first five numbers is 54 and that of the last three numbers is 52. Then the sixth number is?
A. 30
B. 34
C. 24
D. 44

4. The average age of 8 persons in a committee is increased by 2 yrs when two men aged 35 years and 45 years are substitutd by two women . Find the average age of the two women.
A. 48
B. 45
C. 51
D. 42

5. The sum of eight consecutive even numbers of set A is 376. What is the sum of different set of five consecutive numbers whose lowest number is 15 more than the mean of set-A?
A. 296
B. 320
C. 324
D. 284

ANSWERS AND SOLUTION:
1.(C). 1 feet = 12 inch
Total height of Team A = 20 x (5feet + 11inches)
= 20 x (5x12+11)= 1420 inches
Similarly, Total height of team B= 18x(6 feet+2 inches)
= 18 x 74= 1332 inches
Average age of team A and B = (1420+1332)/38 = 72.42

2.(D) Sum of ages of 14 girls and teacher = 15x15= 225 yrs.
So, sum of ages of 14 girls = 14x14 = 196 yrs.
so Teacher's age 225 - 196= 29 yrs.

3.(c) Sum of nine numbers = 9 x 50 = 450
sum of first five numbers = 5 x 54 = 270
sum of last three numbers = 3 x 52 = 156
So sixth number = 450 - 270 - 156  = 24

4.(A) Let average age of 8 people be x years.
According to question,
Sum of age of 7 men + 35 + 45 = 8x
sum of age of 7 men = 8x-80
Let sum of age of two new women be y years
Now,
sum of age of 7 men + y = 8(x+2)
8x - 80 + y = 8x + 16 => y = 96 yrs
Reuired average age = y/2 = 96/2 = 48 yrs.

5.(B) Mean of set A = 376/8 = 47
The lowest number of second set= 47 + 15 = 62
So Required = 62+63+64+65+66= 320