百分率（パーセント）のはじめ

The word 'per cent' really means 'per hundred'. So $1\%$ means '1 out of 100', which is the same as $\frac{1}{100}$. For example, if $37$ squares out of a 100-square grid are coloured in, that is $37\%$. And $100\%$ means the whole thing — one complete amount.

Some percentages are very quick to find. $50\%$ is just half (divide by $2$), $25\%$ is a quarter (divide by $4$), and $10\%$ is simply dividing by $10$. For example, $50\%$ of $80$ is $80÷2=40$; $25\%$ of $60$ is $60÷4=15$; and $10\%$ of $70$ is $70÷10=7$.

Once you can find $10\%$, you can build other percentages. Find $10\%$ first (divide by $10$), then multiply. For example, $30\%$ is three lots of $10\%$, and $5\%$ is half of $10\%$. For $30\%$ of $200$: $10\%$ is $200÷10=20$, so $20\times 3=60$. Building up from $10\%$ is a handy way to work percentages out in your head.

The same amount can be written three ways: as a fraction, a decimal, or a percentage. For example, 'one half' is $\frac{1}{2}=0.5=50\%$. In the same way, $\frac{1}{4}=0.25=25\%$ and $\frac{1}{10}=0.1=10\%$. Others are $\frac{1}{5}=0.2=20\%$, $\frac{3}{4}=0.75=75\%$ and $\frac{1}{100}=0.01=1\%$. They are just different ways of writing the same amount.