Percent means 'out of 100' and is denoted by the symbol %. Percentages are closely related to fractions and decimals.
To change a fraction to a percentage, divide the numerator by the denominator and multiply by 100 - move the decimal point two places to the left. To change a decimal to a percentage, move the decimal point two places to the right.
Some common percentage amounts and their fraction and decimal equivalients.
Percentage |
Fraction |
Reduced fraction |
Decimal |
|---|---|---|---|
$75\%$ |
$\tfrac{75}{100}$ |
$\tfrac{3}{4}$ |
$0.75$ |
$50\%$ |
$\tfrac{50}{100}$ |
$\tfrac{1}{2}$ |
$0.5$ |
$25\%$ |
$\tfrac{25}{100}$ |
$\tfrac{1}{4}$ |
$0.25$ |
$10\%$ |
$\tfrac{10}{100}$ |
$\tfrac{1}{10}$ |
$0.1$ |
$5\%$ |
$\tfrac{5}{100}$ |
$\tfrac{1}{20}$ |
$0.05$ |
To convert a fraction into a percentage, evaluate the fraction and multiply the result by $100$.
To convert a percentage into a fraction, write the percentage as a fraction with $100$ as the denominator and write this fraction in its simplest form.
Write $\frac{3}{5}$ as a percentage.
Dividing $3$ by $5$ gives $0.6$ and $0.6 \times 100=60$. Thus $\frac{3}{5}=60\%$.
Convert $75\%$ into a fraction.
Writing $75$ as a fraction with $100$ as the denominator gives $\frac{75}{100}$. The highest common factor of $75$ and $100$ is $25$. Dividing top and bottom by $25$ gives $\frac{3}{4}$ so we have $75\%=\frac{3}{4}$.
To find $X\%$ of $Y$, the calculation is: \[\frac{X}{100}\times Y\] This is the best method to use if the numbers are difficult - it can just be typed into a calculator.
It is useful to note that:
These can then be used to find more difficult percentages, for example to find $35\%$, find $25\%$ and $10\%$ and add them together, or $2\%$ can be found by finding $1\%$ and doubling it. This is the best method if you can't use a calculator, and often quicker than using a calculator, if it is a simple question.
Find $12\%$ of $£25.40$.
Method 1
Using the formula on a calculator we have:
\[\frac{12}{100} \times 25.40 = 3.048\]
This number must be written $2$ decimal places since it is money. Rounding $3.048$ to $2$d.p. gives $3.05$. So $12\%$ of $£25.40$ is $£3.05$.
Method 2 Using the table above, we have:
$10\% \text{ of }25.40 = 2.54$, and
$1\% \text{ of }25.40= 0.254$
Adding $10\%$ of $25.40$ to two lots of $1\%$ of $25.40$ gives $12\%$ of $25.40$:
\[2.54 + 0.254 + 0.254 = 3.048.\]
So $12\%$ of $£25.40$ is $£3.05$.
If we know that $Z$ is $X\%$ of a number and we want to know what that number is, we multiply $Z$ by $100$ and divide by $X$:
\[\frac{Z\times100}{X}\]
The cost of a computer is $£699$ including VAT ($17.5\%$). Calculate the cost before VAT.
$£699$ is the cost of the original $100\%$ plus the VAT of $17.5\%$ which is a total of $117.5\%$ of the original price.
Start by multiplying $699$ by $100$ to get $69,900$. Then divide $69,900$ by $117.5$. To $2$ decimal places (since we are working with money) this gives $594.89$, so the cost before VAT is $£594.89$.
The cost of a coat which has been reduced by $15\%$ in the sale is $£127.50$. What was the original cost of the coat?
The (reduced) price $£127.50$ is $100\%-15\%=85\%$ of the original amount. To obtain the original amount, we must therefore multiply $127.50$ by $100$ and then divide the result by $85$: \begin{align} &127.50\times100=12,750 &\frac{12,750}{85}=150 \end{align}
So the original cost of the coat is $150$.
Suppose $a$ and $b$ are numbers. To express $a$ as a percentage of $b$ we divide $a$ by $b$ to produce a fraction and use the above rule to convert this fraction into a percentage.
Write $4$ as a percentage of $25$.
Dividing $4$ by $25$ gives $\frac{4}{25}$. To express this fraction as a percentage we must first evaluate the fraction and then multiply the result by $100$: \begin{align} &\frac{4}{25}=0.16\\ \text{and }&0.16\times100=16 \end{align}
so $4$ is equal to $16\%$ of $25$.
A percentage change is a way of expressing a change in a value or quantity. In particular, the percent change expresses the change from the “old” to the “new” value as a percentage of the old value.
Let $v_0$ denote the old value and $v_1$ denote the new value. Then the percentage change from $v_0$ to $v_1$ is given by: \[\textbf{Percentage change }=\frac{v_0-v_1}{v_0}\times100\]
Four years ago, a house was bought for $£180,000$. It is now valued at $£350,000$. Calculate the percentage increase in the value of the house to the nearest $1\%$.
Using the formula we have: \begin{align} \text{Percentage increase} &= \frac{350,000 - 180,000}{180,\!000} \times 100\\ &=\frac{170,000}{180,000} \times 100\\ &= 94\% \text{ to the nearest }1\% \end{align}
A car cost $£12,000$. After three years, it is now worth $£8,000$. Calculate the percentage decrease to the nearest $1\%$.
Using the formula we have: \begin{align} \text{Percentage decrease} &= \frac{12,000-8,000}{12,000} \times 100\\ &=\frac{4,000}{12,000} \times 100\\\ &= 33\% \text{ to the nearest} 1\% \end{align}
Using the percent button on a calculator can have a different effect depending on where it is used. If you are unsure, do not use it, and just use the formula above.
a) 48 ÷ 400 % = gives $12$. So $48$ is $12\%$ of $400$.
b) 1 ÷ 2 × 300 % = gives $1.5$. So $300\%$ of $\frac{1}{2}$ is $1.5$.
c) 400 × 50 % = gives $200$. So $400\%$ of $50$ is $200$.
d) 50 % × 400 = gives $20,000$.But $50\times400=20,000$ so here pressing the % button has no effect.
This workbook produced by HELM is a good revision aid, containing key points for revision and many worked examples.