Working with Proportion (Edexcel IGCSE Maths A (Modular)): Revision Note

Working with proportion

What is direct proportion?

• Direct proportion

  • As one quantity increases/decreases by a certain rate (factor)

  • The other quantity will increase/decrease by the same rate 

• The ratio of the two quantities is constant

  • E.g. 2 boxes of cereal is 800 g of cornflakes

  • Doubling the number of boxes of cereal (4 boxes) will double the amount of cornflakes (1600 g)

How do I solve direct proportion questions?

• Read through wordy direct proportion questions carefully

  • Ensure that you understand the context of the question

  • Some questions may tell you the relationship between the two values as a ratio

• Identify the two quantities involved

  • E.g. Hours worked and pay

• Find the factor that you will be increasing/decreasing by

  • This may be given to you in the question, e.g. 'the amount is tripled'

    • The quantity is multiplied by three

  • Alternatively, find the factor by dividing the 'new' quantity by the 'old' quantity

• Multiply the other quantity by this factor to find the required quantity

  • E.g. If three times as many hours are worked, the pay will be three times more in total

• Give your final answer in context

  • Round and give units where appropriate

What is the unitary method?

• The unitary method means finding one of something (1 unit of something)

  • This can be a useful strategy

• For example, find the weight of 7 boxes, if 8 boxes weigh 60 kg

  • Find the weight of 1 box (1 unit) using division

    • 60 kg ÷ 8 boxes = 7.5 kg per box

  • Scale this unit up using multiplication

    • 7.5 kg per box × 7 boxes = 52.5 kg