To find a fraction of a quantity, divide the quantity by the denominator, then multiply by the numerator. This works because a fraction describes how many equal parts of the whole you are taking.

$$
\text{fraction of a quantity} = \frac{\text{numerator}}{\text{denominator}} \times \text{quantity}
$$

For example, to find \(\frac{3}{5}\) of \(40\):

$$
40 \div 5 = 8
$$

$$
8 \times 3 = 24
$$

So \(\frac{3}{5}\) of \(40\) is \(24\).

To find a percentage of a quantity, convert the percentage to a decimal or fraction, then multiply by the quantity.

$$
\text{percentage of a quantity} = \frac{\text{percentage}}{100} \times \text{quantity}
$$

For example, to find \(25\%\) of \(80\):

$$
\frac{25}{100} = 0.25
$$

$$
0.25 \times 80 = 20
$$

Another common method is to find \(10\%\) first, then build up. This is useful for mental maths.

$$
10\% \text{ of } 90 = 9
$$

$$
5\% \text{ of } 90 = 4.5
$$

$$
15\% \text{ of } 90 = 13.5
$$

Always check that your answer is sensible. For example, a fraction or percentage less than \(1\) (or less than \(100\%\)) should give a result smaller than the original quantity.