FACTS  AND  FORMULAE  FOR  AVERAGE QUESTIONS

 

 

1. Average =Sum of observationsNo. of observations

2. Suppose a man covers a certain distance x kmph and an equal distance at y kmph. Then, the average speed during the whole journey is2xyx+yKmph

Q:

The average age of a class is 19 years. While the average age of girls is 18 and that of boys is 21. If the number of girls in the class is 16, Find the number of Boys in the class?

A) 12 B) 10
C) 8 D) 6
 
Answer & Explanation Answer: C) 8

Explanation:

Let the number of boys = x

From the given data,

=> [21x + 16(18)]/(x+16) = 19

=> 21x - 19x = 19(16) - 16(18)

=> 2x = 16

=> x = 8

Therefore, the number of boys in the class = 8.

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Q:

There are 100 students in 3 sections A, B and C of a class. The average marks of all the three sections was 84. The average of B and C was 87.5 and the average marks of students in A section was :

A) 20 B) 35
C) 54 D) 40
 
Answer & Explanation Answer: A) 20

Explanation:

Numbers of students in section A = x
∴ Numbers of students in section B and C = (100 – x)
∴ x 70 + (100 – x) 87.5 = 84 100
=> 70x + 87.5 100 – 87.5x = 8400
=> 8750 – 17.5x = 8400
=> 17.5x = 8750 – 8400 => x = 20.

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Q:

Third proportion of 10 and 20 is

A) 30 B) 40
C) 25 D) 20
 
Answer & Explanation Answer: B) 40

Explanation:

The third proportional of two numbers p and q is defined to be that number r such that

p : q = q : r.

 

Here, required third proportional of 10 & 20, and let it be 'a'

=> 10 : 20 = 20 : a

10a = 20 x 20

=> a = 40

 

Hence, third proportional of 10 & 20 is 40.

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Q:

8 years ago there were 5 members in the Arthur's family and then the average age of the family  was 36 years. Mean while Arthur got married and gave birth to a child. Still the average age of his family is same now. What is the age of his wife at the time of his child's birth was.If the difference between the age of her  child and herself was 26 years.

A) 25 years B) 26 years
C) 20 years D) can't be determined
 
Answer & Explanation Answer: B) 26 years

Explanation:

Since we know that the difference b/w the age of any two persons remains always constant, while the ratio of their ages gets changed as the time changes.

 

so, if the age of his child be x (presently)

 

Then the age of  wife be x + 26 (presently)

 

Thus the total age = x + ( x + 26) = 32  [252-220 =32]

=> x = 3

Therefore, The age of her child is 3 years and her self  is 29 years. Hence her age at the time of the birth of her child was 26 years.

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Q:

The median of the numbers 3, 1, 1 5, 2 and 7 is

 

A) 3.5 B) 2.5
C) 2 D) 3
 
Answer & Explanation Answer: B) 2.5

Explanation:
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Q:

Amithab's average expenditure for the January to June is Rs. 4200 and he spends Rs.1200 in January and Rs.1500 in july.The average expenditure for the months of febraury to July is:

A) Rs. 4250 B) Rs. 4520
C) Rs. 4060 D) None of these
 
Answer & Explanation Answer: A) Rs. 4250

Explanation:

Amithab's total expenditure for Jan - June  = 4200 x 6 = 25200

 

Expenditure for Feb - June = 25200-1200 = 24000

 

Expenditure for the months of Feb - July = 24000 + 1500 =25500

 

The average expenditure = 25500/6 = 4250

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Q:

The average of 9 results is 50. The average of first four results is 52 and average of last four results is 49 . what is the fifth result?

Answer

Total of results = 50 x 9 = 450


Total of first four results = 52 x 4 = 208


Total of Last four results = 49 x 4 = 196


Hence the fifth result = 450 - (208 + 196) = 46

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Q:

Five boxes of bananas sell for Rs. 30. how many boxes can you buy for Rs.9?

A) 2 B) 1.5
C) 1.25 D) 2.5
 
Answer & Explanation Answer: B) 1.5

Explanation:

Given Five boxes of bananas sell for Rs. 30.

=> 1 Box of Bananas for = 30/5 = Rs. 6

 

Then, for Rs. 9

=> 9/6 = 3/2 = 1.5

 

Hence, for Rs. 9, 1.5 box of bananas can buy.

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