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:

In some quantity of ghee, 60% is pure ghee and 40% is vanaspati. If 10 kg of pure ghee is added, then the strength of vanaspati ghee becomes 20%. The original quantity was : 

A) 10 kg B) 15 kg
C) 20 kg D) 25 kg
 
Answer & Explanation Answer: A) 10 kg

Explanation:

Let the original quantity be x kg. Vanaspati ghee in x kg  =  (40x / 100 )kg = (2x / 5) kg. 

Now, (2x/5)/(x + 10) = 20/100

=> 2x / (5x + 50) = 1/5 

=> 5x = 50 

=> x = 10.

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55 22211
Q:

What will come in place of the question mark (?) in the following question ?

52.5% of 800 + 30.5% of 2800 = ? + 87.30

A) 1096.30 B) 1226.70
C) 1124.20 D) 1186.70
 
Answer & Explanation Answer: D) 1186.70

Explanation:

52.5% of 800 + 30.5% of 2800 = ? + 87.30

420 + 854 - 87.30 = ?

1274 - 87.30 = ?

? = 1186.70

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10 22153
Q:

How many litres of pure acid are there in 8 litres of a 20% solution ?

A) 1.4 B) 1.5
C) 1.6 D) 1.7
 
Answer & Explanation Answer: C) 1.6

Explanation:

Quantity of pure acid = 20% of 8 litres = ((20/100)*8) litres = 1.6 litres.

 

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37 21691
Q:

In a competitive examination in State A, 6% candidates got selected from the total appeared candidates. State B had an equal number of candidates appeared and 7% candidates got selected with 80 more candidates got selected than A. What was the number of candidates appeared from each State ?

A) 4000 B) 8000
C) 12000 D) 16000
 
Answer & Explanation Answer: B) 8000

Explanation:

Let the number of candidates appeared from each state be x.

In state A, 6% candidates got selected from the total appeared candidates

In state B, 7% candidates got selected from the total appeared candidates

But in State B, 80 more candidates got selected than State A

From these, it is clear that 1% of the total appeared candidates in State B = 80

=> total appeared candidates in State B = 80 x 100 = 8000

=> total appeared candidates in State A = total appeared candidates in State B = 8000

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55 21227
Q:

A district has 64000 inhabitants. If the population increases at the rate of 2 * 1/2 % per annum, then the number of inhabitants at the end of 3 years will be : 

A) 65380 B) 68921
C) 70987 D) 72345
 
Answer & Explanation Answer: B) 68921

Explanation:

Population afer 3 years = 64000 * (1 +  5/(2 *100) )^3 = (64000 * 41/40 * 41/40 * 41/40 ) = 68921..

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59 20903
Q:

A number is increased by 20% and then decreased by 20%, the final value of the number is ?

A) increase by 2% B) decrease by 3%
C) decrease by 4% D) increase by 5%
 
Answer & Explanation Answer: C) decrease by 4%

Explanation:

Here, x = 20 and y = - 20
Therefore, the net % change in value
=x + y + xy100 %
20 + -20 + 20 × -20100%  or  - 4%

Since the sign is negative, there is a decrease in value by 4%.

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35 20693
Q:

 The difference between a number and its two-fifth is 510. What is 10% of that number ?

A) 75 B) 85
C) 95 D) 105
 
Answer & Explanation Answer: B) 85

Explanation:

Let the number be x. Then, x-(2/5)x = 510

=> 3x/5 =510 

=>x =[510 * ( 5/3)] =850 

10 % 0f 850 = 85.

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37 20484
Q:

A student multiplied a number by 2/5 instead of 5/2. What is the percentage error in evaluation ?

A) 52% B) 64%
C) 84% D) 77%
 
Answer & Explanation Answer: C) 84%

Explanation:

Let the number be 'x'

 

Then, according to the given data,

 52x-25x52xx100

 

2125x100

 

= 84%

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15 20252