Here is a class lined up for a photograph. In this class there are 12 boys and 18 girls. How would you describe the split between boys and girls to someone who could not see them? Is there a shorter way to say it than "twelve boys and eighteen girls"?

The interactive shows four ratios as coloured bars. Look at the lengths of the bars for each ratio, and how the two groups compare. Read each ratio aloud with the colon as you go.

Here is what each one is showing:

• 2:3 boys to girls: in a class of 30, that is 12 boys and 18 girls.
• 1:4 teachers to pupils: in a group of 30, that is 6 teachers and 24 pupils.
• 3:5 red to blue counters: in a pile of 8, that is 3 red and 5 blue.
• 1:1 cats to dogs: in a shelter of 30, that is 15 cats and 15 dogs.

Today we set the bars to match a story. Before each one, guess which bar will be longer, and by roughly how much.

We work through these four in order:

• 2:1 two dogs for every cat in the shelter
• 3:4 three apples for every four oranges in the fruit bowl
• 2:5 two adults for every five children on the bus
• 1:6 one goalkeeper for every six outfield players in the squad

Optional stretch — a three-part ratio: so far every ratio has compared just two groups. A three-part ratio compares three groups at once. We write it 1:3:6 and read it "one to three to six" — for every 1 of the first colour, there are 3 of the second colour and 6 of the third colour. If your group is ready, use a third colour and build 1:3:6: one counter of the first colour, three of the second, six of the third.