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:

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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38 22411
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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54 22079
Q:

What percent of a day is 3 hours?

A) 14.9% B) 12.5%
C) 15.6%% D) 11.4%
 
Answer & Explanation Answer: B) 12.5%

Explanation:

Let p% of a day is 3 hrs.

We know that a day has 24 hrs.

=> p x 24/100 = 3

=> p = 300/24

=> p = 12.5%

 

Hence, 3 hrs is 12.5% of a day.

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24 21701
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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40 21695
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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57 21271
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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34 21101
Q:

An agent, gets a commission of 2.5% on the sales of cloth. If on a certain day, he gets Rs. 12.50 as commission, the cloth sold through him on that day is worth

A) 300 B) 500
C) 700 D) 900
 
Answer & Explanation Answer: B) 500

Explanation:

Let the total sale be Rs. x.

 

Then, 2.5%. of x = 12.50

=> (25/100 * 1/100 * x) =  125/10

=> x = 500.

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31 21012
Q:

The ratio of earnings of A and B is 4:5. If the earnings of A increase by 20% and the earnings of B decrease by 20%, the new ratio of their earnings becomes 6:5. What are A's earnings?

A) Rs.22,000 B) Rs.27,500
C) Rs. 26,400 D) Cannot be determined
 
Answer & Explanation Answer: D) Cannot be determined

Explanation:
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