Probability as a Decimal and Percentage

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1 Getting Started
2 Watch and Notice
3 Write It Three Ways in Your Copy
4 Try It Together
5 Class Challenge
Pupil practice

1 - Getting Started ~4 mins

Illustration for Getting Started Picture a spinner split into ten equal segments, like a prize wheel at a school fair. Each spin is just as likely to stop on any one of them.

Key point

If we say the chance of landing on one segment is 1/10, what would that same chance look like written as a decimal? And as a percentage?

Teacher Notes

Sketch or picture the ten-segment wheel on the board as you pose this. Take two or three hands-up answers, not open call-outs. Some pupils will already know 1/10 is 0.1; hold out for the percentage too. Don't confirm yet — the lesson is about to show all three side by side. Give a few seconds of quiet think-time before any hands go up.

👇 Mark your steps as done as you complete them.

+5 XP

2 - Watch and Notice ~8 mins

The interactive shows four fair spinners in turn — a ten-segment wheel, a four-segment wheel, an eight-segment wheel, and a fair coin. For each one we read the chance of a single segment three ways: as a fraction, a decimal, and a percentage. Watch how we get from one form to the next, and try reading the next form before it appears.

Teacher Notes

Same move each time: divide top by bottom for the decimal, then times by 100 for the percentage (out of one becomes out of a hundred).

Ten segments: ask for the decimal first — 1/10 = 0.1 = 10%, the friendly one.

Four segments: 1/4 = 0.25 = 25%, the quarter they know from money.

Eight segments: 1/8 = 0.125 = 12.5%. Slow down here — 1 ÷ 8 runs to three decimal places before it stops exactly, further than the others but it still ends; note the half a percent.

Coin: 1/2 = 0.5 = 50%, the fifty-fifty they meet first.

Then work two written conversions slowly on the board: 1/8 (1 ÷ 8 = 0.125, then × 100 = 12.5%) and 3/5 (3 ÷ 5 = 0.6, then 0.6 × 100 = 60%, so 3/5 = 0.6 = 60%).

Flag the 3/5 one — it returns in the Class Challenge.

Reading and naming the wheels is brisk; the two written conversions are where you take your time.

👇 Mark your steps as done as you complete them.

+10 XP

3 - Write It Three Ways in Your Copy ~3 mins

COPYBOOK MOMENT

In your maths copy, write each of these probabilities three ways in a row: the fraction first, then the decimal, then the percentage. Keep one set under the other so they line up.

1/10
1/4
1/8
1/2

Then circle the form you find easiest to compare with.

Teacher Notes

Walk the room glancing for the three-column line-up and the divide-then-times-100 working — this is whole-class copybook practice, not marking. Watch for the 1/8 row, where the decimal should have three places.

+10 XP

4 - Try It Together ~10 mins

Today we work this ten-segment wheel together — think of it as a school-fair prize wheel where some spaces win and some don't.

Key point

A prime number has exactly two factors: 1 and itself. On this wheel, 2, 3, 5 and 7 are the primes (each divides only by 1 and itself), so four of the ten spaces are winners. That makes the chance of landing on a prime 4/10.

Before we spin, say the chance of a prime three ways: 4/10, then the decimal, then the percentage. Then we record whether each spin is a hit (a prime) or a miss, and see how the run compares to our 4/10 prediction.

Chance of a prime: 4/10 = 0.4 = 40%

Teacher Notes

Talk this one through together — pupils take turns at the board and the class agrees or corrects out loud.

Hold out for all three forms of 4/10 before the first spin: 4/10 = 0.4 = 40%. Keep a hit/miss tally on the board yourself as pupils spin; nobody needs their own device here — the rest of the class watches and calls the next outcome. Revoice a strong answer: so four out of ten is the same as forty out of a hundred. Don't expect the run of spins to land on exactly 40% — name that wobble aloud, it seeds the maths-talk.

+15 XP

5 - Class Challenge ~8 mins

Key point

Now we convert probabilities on the battery slider. Think of the battery as a percentage bar: an empty battery is 0% and a full battery is 100%, so wherever the slider sits IS the percentage we are converting to. To show 1/10, for example, we fill the battery to one tenth of full, which is 10%.

Each round gives a fraction. Set the battery slider to the matching percentage. We'll work these in order: 1/10, then 1/4, then 3/8, then 3/5, the one we just worked out as 0.6 = 60%, and finally 7/10.

Convert the probability

Teacher Notes

This is the practice round — pupils take turns at the board, check each answer, and the class confirms before moving on. Keep the board work brisk rather than over-explaining.

The practice set rises: 1/10 and 1/4 are recall, 3/8 forces the three-place decimal and the .5% percentage, 3/5 = 0.6 = 60% was worked on the board in Watch and Notice so pupils have it to lean on, and 7/10 = 0.7 = 70% tests whether they read tenths straight off. Before each Check, ask the class to predict the percentage. Revoice: 3 divided by 8 is 0.375, and 0.375 times a hundred is 37 and a half per cent.

+15 XP

Pupil practice

Module 7 · Data, Chance and the Co-ordinate Plane Data & Chance

Lesson 94 · Probability as a Decimal and Percentage

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This lesson is copyright of Coding Ireland 2017 - 2025. Unauthorised use, copying or distribution is not allowed.

Teacher Class Feed

1 - Getting Started ~4 mins

2 - Watch and Notice ~8 mins

3 - Write It Three Ways in Your Copy ~3 mins

4 - Try It Together ~10 mins

Chance of a prime: 4/10 = 0.4 = 40%

5 - Class Challenge ~8 mins

Convert the probability

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