How to calculate percentages
What is a percent?
The word ‘percentage’ is derived from the Latin phrase ‘per centum’, which means ‘by the hundred’. Percentages are fractions with a denominator of $$100$$. Putting in another way, it is the relationship between the portion and the whole in which the whole is always valued at $$100$$. A percentage is a fraction of a ratio in which the full value is always $$100%$$. For example, if Sam received a $$60%$$ on his math test, it implies that he received a $$30$$ mark out of a possible $$100$$. In fraction form, it is $$\frac{60}{100}$$ and in ratio terms, is $$60:100$$.
Percentage definition
A part or an amount in every hundred is defined as a percentage. It is a fraction having a denominator $$100$$ and is denoted by the sign $$\%$$.
E1.12: Calculate the given percentage of a quantity
There are three possible unknowns or variables in any percentage problem:
Percentage
Part
Base
You must be able to recognise these factors to solve any percentage problem. Take a look at the examples below. Each of the three variables is known:
For example, if $$20%$$ of $$200$$ is $$40$$, $$20$$ is the percentage, $$200$$ is the base and $$40$$ is the part.
The term ‘percentage’ is another term for ‘hundredths’. As a result, $$1%$$ equals on hundredth, i.e., $$1\%=\frac{1}{100}=0.01$$.
Percentage formula
To find the percentage of a whole in terms of $$100$$, use the percentage formula. You can represent a number as a fraction of $$100$$ using this formula.
$$\text{Percentage}=\frac{Value}{Total value}\times 100$$
For example, out of $$40$$ pens, the number of red cells is $$8$$. Hence, out of $$100$$, the number of red cells will be $$\frac{8}{40}\times 100=20%$$.
Worked examples
Example 1: Find the percentage of the following:
What is the percentage of $$5$$ out of $$50$$?
What is the percentage of $$20$$ out of $$200$$?
What is the percentage of $$40$$ out of $$50$$?
Question 1
Step 1: Write the formula.
$$\text{Percentage}=\frac{Value}{Total value}\times 100$$
Step 2: Write the given information.
$$\text{Value}=5$$ and $$\text {Total value}=50$$.
Step 3: Substitute the given value in the formula.
$$\frac{5}{50}\times 100=10\%$$
So, the percentage is $$10\%$$.
Question 2
Step 1: Write the formula.
$$\text{Percentage}=\frac{Value}{Total value}\times 100$$
Step 2: Write the given information.
$$\text{Value}=20$$ and $$\text{Total value}=200$$
Step 3: Substitute the given value in the formula.
$$\frac{20}{200}\times 100=10\%$$
So, the percentage is $$10\%$$.
Question 3
Step 1: Write the formula.
$$\text{Percentage}=\frac{Value}{Total value}\times 100$$
Step 2: Write the given information.
$$\text {value}=40$$ and $$\text {total value}=50$$.
Step 3: Substitute the given value in the formula.
$$\frac{40}{50}\times 100=80\%$$.
So, the percentage is $$80\%$$.
E1.12A: Calculate the given percentage of a quantity
To calculate the percentage, divide the value by the total value and multiply the result by $$100$$. It is represented as:
$$\text{Percentage} =\frac{Value}{Total value} \times 100$$
For example: $$\frac{2}{5}\times 100=0.4\times 100 = 40\%$$.
How to calculate the percentage of a quantity
We must use a separate formula to determine the percentage of a number, that is:
$$P\%$$ is equal to $$X$$, where $$X$$ is the required percentage.
If we want to remove the $$\%$$ sign, we need to express the above formula as:
$$\frac {P}{100}\times \text{Number} = X$$
For example: Calculate $$10\%$$ of $$80$$.
Let $$10\%$$ of $$80$$ is equal to $$X$$
$$\frac {10}{100}\times 80 =X$$
$$X=8$$
Worked examples
Example 1: Find the percentage of the following.
If $$10%$$ of $$40\%$$ of a number is $$10$$, what is the number?
If $$40%$$ of $$60\%$$ of a number is $$20$$, what is the number?
Question 1
Step 1: Write the formula.
$$P\%$$ of a number is equal to $$X$$.
Step 2: Write the given information.
Here, $$P=\frac{10}{100}\times \frac{40}{100}$$
$$X=10$$
Step 3: Substitute the given value in the formula.
$$\frac{10}{100}\times \frac{40}{100}\times\text{Number}=10$$
$$\text{Number}=\frac{10\times 100\times 100}{10\times 40}$$
$$\text{Number}=250$$
So, the number is $$250$$.
Question 2
Step 1: Write the formula.
$$P\%$$ of a number is equal to $$X$$.
Step 2: Write the given information.
Here, $$P=\frac{40}{100} \times \frac{60}{100}$$
$$X=20$$
Step 3: Substitute the given value in the formula.
$$\frac{40}{100}\times \frac{60}{100}\times \text{Number}=20$$
$$\text{Number}=\frac{20\times 100\times 100}{40\times 60}$$
$$\text{Number}=8.33$$
So, the number is $$8.33$$.
So, the number is $$8.33$$.
E1.12B: Express one quantity as the percentage of another and how to calculate the percentage of a quantity
$$P\%$$ of a number corresponds to $$X$$.
Here, $$X$$ is the required percentage.
If you remove the $$%$$ sign, you need to write the above formula as:
$$\frac{P}{100} \times \text{Number}=X$$
For example: Calculate $$20%$$ 0f $$80$$.
Let $$20%$$ of $$80$$ be $$X$$.
$$\frac {20}{100}\times 80$$ is $$X$$.
So, $$X=16$$.
Percentage chart
The percentage chart for fractions converted into percentages is given here:
Worked examples
Example 1: Find the percentage of the following quantities.
Find $$30%$$ of $$150$$.
Find $$20%$$ of $$2\text{hours}$$.
Find $$15%$$ of $$150$$.
Question 1
Step 1: Write the formula.
$$P\%$$ of a number is equal to $$X$$.
Step 2: Write the given information.
We need to find $$30\%$$ of $$150$$.
$$P=30$$
$$\text{Number}=150$$
Step 3: Substitute the given value in the formula.
$$\frac{30}{100}\times 150=X$$
$$X=3\times 15$$
$$X=45$$
Therefore, $$X=45$$.
Question 2
Step 1: Write the formula.
$$P\%$$ of a number is equal to $$X$$.
Step 2: Write the given information.
We need to find $$20\%$$ of $$2\text{hours}$$.
$$P=30$$
Since, $$2\text{hours}=120\text{minutes}$$.
$$\text{Number}=120$$
Step 3: Substitute the given value in the formula.
$$\frac{20}{100}\times 120=X$$
$$X=2\times 12$$
$$X=24$$
Therefore, $$X=24\text {minutes}$$.
Question 3
Step 1: Write the formula.
$$P\%$$ of a number is equal to $$X$$.
Step 2: Write the given information.
$$P=15$$
$$\text{Number}=150$$
Step 3: Substitute the given value in the formula.
$$\frac{15}{100}\times 150=X$$
$$X=1.5\times 15$$
$$X=22.5$$
Therefore, $$X=22.5$$.