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 an election between two candidates, one got 55% of the total valid votes, 20% of the votes were invalid. If the total number of votes was 7500, the number of valid votes that the other candidate got, was :

A) 2500 B) 2700
C) 2900 D) 3100
 
Answer & Explanation Answer: B) 2700

Explanation:

Total number of votes = 7500 

Given that 20% of Percentage votes were invalid

 => Valid votes = 80%

 Total valid votes = 7500*(80/100) 

1st candidate got 55% of the total valid votes. 

Hence the 2nd candidate should have got 45% of the total valid votes 

=> Valid votes that 2nd candidate got = total valid votes x (45/100) 

7500*(80/100)*(45/100) = 2700

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745 109465
Q:

If 20% of a = b, then b% of 20 is the same as :

A) 4% of a B) 6% of a
C) 8% of a D) 10% of a
 
Answer & Explanation Answer: A) 4% of a

Explanation:

20% of a = b  => (20/100)a = b 

 

b% of 20 =(b/100) x 20 = (20a/100) x (1/100) x (20) = 4a/100 = 4% of a.

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Q:

Fresh fruit contains 68% water and dry fruit contains 20% water. How much dry fruit can be obtained from 100 kg of fresh fruits ?

A) 20 B) 30
C) 40 D) 50
 
Answer & Explanation Answer: C) 40

Explanation:

The fruit content in both the fresh fruit and dry fruit is the same.

 

Given, fresh fruit has 68% water.so remaining 32% is fruit content. weight of fresh fruits is 100kg

 

Dry fruit has 20% water.so remaining 80% is fruit content.let weight if dry fruit be y kg.

 

Fruit % in freshfruit = Fruit% in dryfruit 

 

Therefore, (32/100) x 100 = (80/100 ) x y 

 

we get, y = 40 kg.

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466 59868
Q:

A student has to obtain 33% of the total marks to pass. He got 125 marks and failed by 40 marks. The maximum marks are :

A) 500 B) 600
C) 800 D) 1000
 
Answer & Explanation Answer: A) 500

Explanation:

Given that the student got 125 marks and still he failed by 40 marks

 

=> The minimum pass mark = 125 + 40 = 165

 

Given that minimum pass mark = 33% of the total mark

 

=> total mark =33/100 =165

 

=> total mark = 16500/33 = 500

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182 50805
Q:

The value of a machine depreciates at the rate of 10% every year. It was purchased 3 years ago. If its present value is Rs. 8748, its purchase price was :

A) 10000 B) 12000
C) 14000 D) 16000
 
Answer & Explanation Answer: B) 12000

Explanation:

Purchase price = Rs.87481-101003 = Rs. [8748 * 10/9 * 10/9 * 10/9] = Rs.12000

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273 50275
Q:

A student multiplied a number by 3/5 instead of 5/3, What is the percentage error in the calculation ?

A) 54 % B) 64 %
C) 74 % D) 84 %
 
Answer & Explanation Answer: B) 64 %

Explanation:

Let the number be x.

Then, ideally he should have multiplied by  x by 5/3. Hence Correct result was x * (5/3)= 5x/3. 

 

By mistake he multiplied x by 3/5 . Hence the result with error  = 3x/5 

Then, error = (5x/3 - 3x/5) = 16x/15 

Error %  = (error/True vaue) * 100 = [(16/15) * x/(5/3) * x] * 100 = 64 %

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183 47909
Q:

A man spends 35% of his income on food, 25% on children's education and 80% of the remaining on house rent. What percent of his income he is left with ?

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

Explanation:

Let the total income be x.

 

Then, income left = (100 -80)% of [100 -  (35 + 25)] % of x = 20% of 40% of x = 20/100 * 40/100 * 100) % of x = 8 % of x.

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142 43374
Q:

if the price of a book is first decreased by 25% and then increased by 20%, then the net change in the price will be  : 

A) 10 B) 20
C) 30 D) 40
 
Answer & Explanation Answer: A) 10

Explanation:

Let the original price be Rs. 100.

New final price  = 120 %  of (75 % of Rs. 100) = Rs. (120/100 * 75/100 * 100) = Rs. 90.

Decrease = 10%

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