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Primary 6 Ratio

A ratio compares equal parts in a stated order. Add its terms when you need the total number of parts.

Reading a ratio from a picture

a : b compares the first group with the second. a : b : c compares three groups in that exact order. Each counter below represents one equal unit.

Red : blue = 2 : 3. There are 2 red parts and 3 blue parts, so there are 5 parts in all. Blue : red would be 3 : 2.
Red : blue : green = 2 : 3 : 4. The order names each group. There are 9 parts in all.

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In the three-colour picture, what is the ratio of blue counters to green counters?

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3 : 4

Equivalent ratios and a missing term

Equivalent ratios describe the same comparison. Multiply or divide every term by the same number. This works for two-part and three-part ratios.

Two-part ratio
Three-part ratio

Find the missing number

3 : 5 = 9 : ?

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4 : 7 = 12 : ?. Find the missing number.

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21

Writing a ratio in simplest form

Divide every term by their highest common factor (HCF). A ratio is in simplest form when no whole number greater than 1 divides all its terms.

Two-part ratio

Write 12 : 18 in simplest form.

Three-part ratio

Write 6 : 9 : 12 in simplest form.

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Write 15 : 20 in simplest form.

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3 : 4

Fractions and ratios

A ratio compares parts with parts. A fraction of the whole compares one part with the total. Add all the ratio terms before writing the denominator.

From a two-part ratio to a fraction

Red : blue = 2 : 3. What fraction of the counters are red?

From a three-part ratio to a fraction

Red : blue : green = 2 : 3 : 4. What fraction of the counters are blue?

From a fraction to a ratio

35 of some beads are red. What is the ratio of red beads to other beads?

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27 of the counters are green. What is green : other counters?

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2 : 5

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