Ratios

• A ratio is a relationship between two or more numbers that can be sometimes expressed as a fraction.
• As with fractions, ratios also need to be written in simplest form, that is divide by all common multiples, for example, \(4:2\) has a common multiple of \(2\), so in simplest form can be written as \(2:1\).
• When using ratios in context, remember that the units need to be the same, for example \(1 \mbox{ cm} :1 \mbox{ m}\) should be written as: 
  \begin{eqnarray*}
  1\mbox{ cm} &:& 1 \mbox{ m} \\
  1 \mbox{ cm}&:& 100 \mbox{ cm}\\
  1 &:& 100\,
  \end{eqnarray*}

Proportions

• A proportion is a special form of an algebra equation.
• It is used to compare two ratios or make equivalent fractions.
• For example, the concentration of some medication is expressed as a proportion, e.g. as \(800\) milligrams in \(10\) millilitres. If we need to give a patient \(300\) milligrams, we need to know what proportion of the \(10\) millilitres will be the correct dose. Once we have the fraction we need to multiply by the volume.
  \[
  \frac{300 \mbox{ mg}}{800 \mbox{ mg}} \times 10 \mbox{ ml}
  = 3.75 \mbox{ ml}
  \]