FACTS  AND  FORMULAE  FOR  PERCENTAGE  QUESTIONS

 

 

I.Concept of Percentage : By a certain percent , we mean that many hundredths. Thus x percent means x hundredths, written as x%.

To express x% as a fraction : We have , x% = x/100.

Thus, 20% = 20/100 = 1/5;     

48% = 48/100 = 12/25, etc.

To express a/b as a percent : We have, ab=ab×100% .

 

Thus, 14=14×100%=25%

   

 

II. If the price of a commodity increases by R%, then the reduction in consumption so as not to increase the expenditure is R100+R×100%

If the price of the commodity decreases by R%,then the increase in consumption so as to decrease the expenditure is R100-R×100%

 

 

III. Results on Population : Let the population of the town be P now and suppose it increases at the rate of R% per annum, then :

1. Population after n years = P1+R100n

 

2. Population n years ago =  P1+R100n

 

 

IV. Results on Depreciation : Let the present value of a machine be P. Suppose it depreciates at the rate R% per annum. Then,

1. Value of the machine after n years = P1-R100n

 

2. Value of the machine n years ago = P1-R100n

 

 

V. If A is R% more than B, then B is less than A by

R100+R×100%    

If A is R% less than B , then B is more than A by 

R100-R×100%  

Q:

Each edge of a cube is increased by 20% then the percentage increase in surface area of the cube is : 

A) 144% B) 40%
C) 44% D) 72.8%
 
Answer & Explanation Answer: C) 44%

Explanation:

The surface area of a cube = 6a2 =  6 side2

 

New  Surface area = 6 x 1.44a2

 

Therefore, 0.44a2a2x100 = 44%

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Q:

5% of a number is 25% more than another number. The difference between the numbers is 96. Then find the value of the numbers?

A) 102 & 6 B) 99 & 3
C) 104 & 8 D) 100 & 4
 
Answer & Explanation Answer: D) 100 & 4

Explanation:

Let the first number be x and the second number be y.

Given

5% of x = (25% of y) + y

=> 5x/100 = 5y/4

=> x = 25y ......(1)

and also given that, x - y = 96

=> x = 96 + y .....(2)

From (1)&(2)

25y = 96 + y

=> 24y = 96

=> y = 4

From (1)

x = 96 + 4 = 100

Hence, the numbers are 100 & 4.

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Q:

If 3/5 of x is equal to 4/9 of y, then what is the ratio of x and y?

 

 

A) 11 : 15 B) 15 : 23
C) 20 : 27 D) 14 : 25
 
Answer & Explanation Answer: C) 20 : 27

Explanation:
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Q:

If 1/8 of a pencil is black, 1/2 of the remaining is white and the remaining, 3/2cm is blue. Find the total length of the pencil?

A) 8 cms B) 7 cms
C) 9 cms D) 11 cms
 
Answer & Explanation Answer: A) 8 cms

Explanation:

Let the total length of the pencil = x cms

From the given data,

'x/8' is black

Remaining part = x - x/8 = 7x/8

1/2 of 7x/8 is white

=> 7x/16 is white (Remaining part is 7x/16)

And given that the remaining part is blue and is 3 1/2 cm = 7/2 = 3.5 cm

That remaining part = 7x/16

Therefore,

7x/16 = 7/2

x = 16/2 = 8

Hence, the total length of the pencil = x = 8 cm.

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Q:

In an election only two candidates contested. A candidate secured 70% of the valid votes and won by a majority of 172 votes. Find the total number of valid votes  ?

A) 446 B) 415
C) 404 D) 430
 
Answer & Explanation Answer: D) 430

Explanation:

Let the total number of valid votes be 'V'.

70% of V = 70/100 x V = 7V/10

Number of votes secured by the other candidate = V - 7V/100 = 3V/10

Given, 7V/10 - 3V/10 = 172 => 4V/10 = 172

=> 4V = 1720 => V = 430.

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Q:

Rs. 69 were divided among 115 family members so that each lady gets 50 paise less than each gentleman, and each gentleman received twice the paise as each lady received. Find the number of  gentlemen in the family?

A) 23 B) 92
C) 48 D) 52
 
Answer & Explanation Answer: A) 23

Explanation:

Total number of female members = 115

Let the number of males = x

Number of females = y

=> x + y = 115  ......(1)

According to the given data,

x + y(0.50) = 69 ......(2)

From (1) & (2), we get

0.5y = 46

=> y = 46 x 2 = 92

=> x = 115 - 92 = 23

 

Hence, required number of gentlemen in the family = x = 23.

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8 7650
Q:

Lucky started working 2 years ago. Her starting salary was half of Karthik's salary at that time. Each year since then Lucky and Karthik have received a rise of 10% in their respective salary. What percentage (to the nearest percent) of Karthik's current salary is Lucky's current salary ?

A) 75% B) 22.5%
C) 35% D) 50%
 
Answer & Explanation Answer: D) 50%

Explanation:

Let the salary of Karthik 2 years ago be Rs.100
So, Lucky's salary 2 years ago was Rs.50

 

Karthik's present salary has seen two 10 percent increases. So as Lucky's salary.

So, Karthik's salary 1 year ago = 100 + 10% of 100 = 100 + 10 = 110.
Karthik's present salary = 110 + 10% of 110 = 110 + 11 = 121.

 

Lucky's salary 1 year ago = 50 + 10% of 50 = 50 + 5 = 55
Lucky's present salary = 55 + 10% of 55 = 55 + 5.5 = 60.5

 

Lucky's present salary of 60.5 = 50% of Karthik's present salary of 121.

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Q:

When the price of whear was increased by 32%, a family reduced its consumption in such a way that the expenditure on wheat was only 10% more than before. If 30 kg per month were consumed before, find the new monthly consumption of family?

A) 27 kgs B) 25 kgs
C) 24 kgs D) 21 kgs
 
Answer & Explanation Answer: B) 25 kgs

Explanation:

Initial wheat per kg cost be Rs. 100

Then, monthly expenditure of the family = 30 x 100 = 3000

After increase in cost,

Let P be new monthly consumption of family.

P x 132 = 110/100 x 3000

=> P = 25 kg.

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