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Maths for SG Primary Schools

Primary 6 maths · Ratio, rate & percentage · About 12 minutes

Find percentage increase and decrease

Mark the starting amount, the change, and which of them the question compares.

Start the interactive lesson →Open practice activity

What you will learn

Models used: number sentences, stories, tables.

Before you start

Compare the change with the start

A plant was 80 cm tall. It grows by 20 cm. The starting 80 cm is our whole.

  1. The complete bar is the whole. Each labelled section is one part of it.

    A plant was 80 cm tall. It grows by 20 cm. The starting 80 cm is our whole.

    From 40 to 36 is a decrease of 4, compared with 40, so it is a 10% decrease.

  2. The full bar represents cm80, which is 100%. The shaded share is 25%.

    20 is one quarter of the original 80.

    Each section: 5% = 4cm

    25% of 80cm = 20cm

  3. The full bar represents cm80, which is 100%. The shaded share is 25%.

    A quarter is 25%. The plant increased by 25%, even though its new height is 100 cm.

    20 ÷ 80 × 100% = 25%

    Each section: 5% = 4cm

    25% of 80cm = 20cm

What did the picture help us see?

A quarter is 25%. The plant increased by 25%, even though its new height is 100 cm.

Worked example: A new example, together

A quantity changes from 40 to 36. What is the percentage decrease?

The original amount is the reference whole. The change is the gap between these values.
Original amountNew amount
4036

  1. The original amount is the reference whole. The change is the gap between these values. Read each value with the heading above its column.

    Mark the starting amount, the change, and which of them the question compares.

    Percentage change compares the change with the original amount.

    The original amount is the reference whole. The change is the gap between these values.
    Original amountNew amount
    4036

  2. The original amount is the reference whole. The change is the gap between these values. Read each value with the heading above its column.

    The change is 4.

    The original amount is the reference whole. The change is the gap between these values.
    Original amountNew amount
    4036
    A quantity changes from 40 to 36. What is the percentage decrease?

    10 %

  3. The original amount is the reference whole. The change is the gap between these values. Read each value with the heading above its column.

    The change is 4.

    The original amount is the reference whole. The change is the gap between these values.
    Original amountNew amount
    4036

  4. The original amount is the reference whole. The change is the gap between these values. Read each value with the heading above its column.

    Use the original 40 as the whole.

    The original amount is the reference whole. The change is the gap between these values.
    Original amountNew amount
    4036

  5. The original amount is the reference whole. The change is the gap between these values. Read each value with the heading above its column.

    4 ÷ 40 × 100% = 10%.

    The original amount is the reference whole. The change is the gap between these values.
    Original amountNew amount
    4036

  6. The original amount is the reference whole. The change is the gap between these values. Read each value with the heading above its column.

    Check the result against the question we started with.

    40 − 10% of 40 = 36.

    We found 10 %. Percentage change compares the change with the original amount.

    The original amount is the reference whole. The change is the gap between these values.
    Original amountNew amount
    4036

Common mistakes to discuss

Check your understanding

Explain your reasoning to an adult, then try a fresh question in the interactive lesson. A result from one attempt is not proof of lasting mastery.

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