Compound Units (SQA National 5 Applications of Mathematics): Revision Note

Compound units

What is a compound unit?

• A compound unit is a measurement that is calculated by using more than one measurement

• Compound units can be used to measure rates

  • This measures how much one quantity changes when the other is increased by 1

  • Examples include:

    • Speed – how much the distance changes for each unit of time

    • Flow rate – how much the volume changes for each unit of time

    • Population density – how many people there are for each unit of area

    • Fuel consumption - volume of fuel used for each unit of distance travelled

How do I calculate quantities using compound units?

• Write the compound unit as a conversion factor

• Consider the compound unit 50 km per litre

  • Rewrite as 1 L = 50 km

  • If you are given the number of litres, then multiply to find the kilometres

    • 3.5 L = 175 km

  • If you are given the number of kilometres, then divide to find the litres

    • 110 km = 2.2 L