Some situations involving percentage can be resolved through a simple rule of three. we mean by percentage one reason centesimal (fraction with denominator equal to 100) which is called ratepercentage and is represented by the symbol % (Percent). For example, if we have 45%, we can represent it in the following ways:
45% = 45
100
or
9
20
or 0.45
Whenever we use the rule of three in order to determine percentages, we must relate the part of the whole with the value of 100%.
Note: In situations involving a percentage, we perform cross multiplication because it is a quantity directly proportional.
Examples
1º) Determine the 95% value of BRL 105.00
100x = 95·105
100x = 9975
x = 9975
100
x = 99.75 reais
Therefore, 95% of BRL 105.00 is equal to BRL 99.75.
2º)In a room of 40 students, a survey was conducted, which showed that 30 students like to play sports. What percentage of students like sports?
40x = 100 * 30
40x = 3000
x = 3000
40
x = 75%
We have that 75% of the students in this class like sports.
3º) Pedro got 21 questions right from a test, which correspond to 70% of the total questions. How many questions did the test have?
70x = 21*100
70x = 2100
x = 2100
70
x = 30
The test had 30 questions.
4º) In one promotion, the price of an object was reduced from R$76.00 to R$57.00. Calculate the percentage discount amount.
We must first determine the actual value of the discount: 76 – 57 = 19. By comparing the discount amount with the amount without the discount, we get the percentage amount.
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76x = 100 * 19
76x = 1900
x = 1900
76
x = 25%
The discount given was 25%.
5º)A restaurant bill, including 10% service, was R$143.00. What is the value of the account without the service charge?
110x = 143 * 100
110x = 14300
x = 14300
110
x = 130
The bill without the value of the service is R$ 130.00.
6º)A product that cost R$ 80.00 was readjusted by 25%. Determine the new product value.
100x = 125 * 80
100x = 10000
x = 100
The price of the product after the readjustment is R$ 100.00.
7º)The price of a computer is R$ 2,200.00. What will the price of the computer be if it undergoes an 18% readjustment?
100x = 2200 * 118
100x = 259600
x = 259600
100
x = 2,596
If the 18% readjustment happens, the computer will cost R$ 2 596.00.
8º)Considering that the population of a country is around 180 million inhabitants and 38 million are considered smokers, what is the percentage of smokers in that country?
180x = 3800
x = 3800
180
x = 21.1
The percentage of smokers in that country is approximately 21.1%.
by Mark Noah
Graduated in Mathematics
Would you like to reference this text in a school or academic work? Look:
SILVA, Marcos Noé Pedro da. "Percentage calculation with rule of three"; Brazil School. Available in: https://brasilescola.uol.com.br/matematica/porcentagem-utilizando-regra-tres.htm. Accessed on June 27, 2021.
