Percentage Formulas, Shortcuts, Tips and Tricks

Percentage Solved Examples - Page 3
Percentage Important Questions - Page 4

Per cent means ‘for every hundred’. To make comparisons easier, we use the concept of percentage.6% (read as six per cent, which means, 6 out of 100 or 6 for every 100) is same as \frac{6}{100} or 0.006 or 6:100. Any percentage value can also be expressed as
(1) an ordinary fraction
(2) a decimal fraction or
(3) a ratio

Value of Percentage: Value of percentage always depends on the quantity to which it refers.

Consider the Statement: “65% of the students in this class are boys”. From the context, it is understand that boys from 65% of the total number of students in the class. To know the value of 65%, the value of the total number of students should be known.

If the total number of students in 200, then the number of boys = \frac{200\times 65}{100} = 130.
Note that the expressions 6%, 63%, 72%, 155%, etc do not have any value intrinsic to themselves. Their values depend on the quantities to which they refer.

Comparing Percentages: Which of three 25%, 5% and 125% is the larger?

It should be remembered that no comparison can be made about the above percentages, because they do not have intrinsic values. If 25% of 10,000, then value is 0.25\times \, 10,000\, =\, 2,500 and if 75% refers to 75% of 100, its value is 0.75\times\, 100\, =\, 75 . We can conclude that 25% of 10,000 > 75% of 100.

Ordinary Fractions and Decimal Fractions: It is already noted that any given percentage can be written either in the form of a decimal fraction or an ordinary fraction. If can remember the conversion of a fraction to a percentage and vice-versa his/her speed of calculations can be improved.

Percentage of Increase/Decrease: If the production of rice in 2003 is 800 million tonnes and in 2004 it is 1000 million tonnes, then the increase in the production of rice from 20003 to 2004 is 200 million tonnes, i.e., an increase of 200 million tonnes over the production of rice in.
2003, i.e., \frac{200}{800}\, =\,\frac{1^{th}}{4} increase = (25%).
Percentage increase of decrease of a quantity is the ratio of the amount increase or decrease in a quantity to the original amount of the quantity, expressed to the base of (or with respect to) 100.

Percentage increase = \frac{Increase\, in \, quantity}{original \, quantity}\times 100

Percentage decrease = \frac{Decrease\, in \, quantity}{original \, quantity}\times 100

Change in quantity = \frac{Percentage \, change}{100}\times original \, quantity

Increase and Decreased Values: If Rathod’s income from Rs,2,000 by 20% then the increase in income
= 2000\times 0.2 = Rs 400

The increase income (i.e., income after increase)
= (initial\, value)\times (1+ \%\, Increase)
= 2000(1+0.2) = Rs 2,400

Successive Percentages: Let the population of a town at the beginning of 2004 be 50,000 and during the year it increased by 20% i.e.., at the beginning of 1992, the population was 5000\times (1+0.2) = 60,000. Suppose, during 1992 it again increased by 25%. This means that at the beginning of 1993, the population was 60,000\times (1+0.25) = 75,000.

To represent situations of the above type, the phrase successive change (\frac{increase}{decrease})etc. is used.

In the case of the above example, we say, “The population of a town which was 50,000 at the beginning of 2004 successively increased by 20% p.a. and 25% p.a. during the next two years”.
In such cases, the final value(1+0.2)(1+0.25)
= 50,000\times 1.2\times 1.25 = 75,000

In general way, if p_{1},p_{2},p_{3}, ……………. are successive percentage of change, then,
Final value = (1\pm p_{1}\%)(1\pm p_{2}\%)(1\pm p_{3}\%) ……….. \times (Initial \,value)

Where ’+’ is used for increases and ‘-‘ is used for decreases)
Note : Even if the changes are mixed group of some increases and some decreases, the above procedure can be applied.

Percentage Points: Let us consider the agricultural production of a state. Suppose, in the year 2003, quantity of wheat produced was 40% of the total food grains produced. And in the year 2004, wheat produced was 50% of the total food grains produced. Suppose we want to measure the percentage change in the production of wheat, calculating percentage increase from 40 to 50 as \frac{50-40}{40}\times 100 and saying it is 25% is not correct.

40%, in the year 2003 refers to 40% of total agricultural production of that year.
Similarly, 50% in the year 2004, refers to 50% the total agricultural production of 2004.

Neither the values of agricultural production of 2003 and 2004 are known, nor is any relation between the two quantities given.

Hence, 40% of the year 2003 and 50% of the 2004 cannot be compared, until the total agricultural production ‘relations is known’.
With the available data we cannot find the percentage \frac{increase}{decrease} in the production of wheat from 2003 to 2004. In such situations, the phrase, ‘percentage points’ is used. “Wheat production (as a percentage of the total agricultural produce) in the year 2004 is 10 percentage points more than that wheat production (as percentage of the total agricultural produce) of the year 2003.

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