Learn the 3-Step Method
1Count ALL the outcomes.
A fair dice has 6 equally likely outcomes: 1, 2, 3, 4, 5, 6.
2Count the favourable ones.
Rolling an even number: 2, 4, 6 — that's 3 outcomes.
3Write the fraction.
P(even) = 3/6 = 1/2 — an even chance.
Worked Example
A bag holds 3 red, 2 blue and 5 green marbles. What is the probability of picking green?
Watch Out! Common Mistakes
Impossible! You can’t have more favourable outcomes than outcomes. Probabilities live between 0 and 1.
Careful! ‘Win or lose — so it’s 50-50’ is wrong unless both are equally likely. The fraction rule needs a fair dice, coin or draw.
The gambler’s trap! Three heads in a row doesn’t make tails ‘due’ — each fair flip stays ½.
⬇ Cheat Sheet
- Scale: 0 impossible → ½ even chance → 1 certain.
- P = favourable ÷ total (equally likely outcomes).
- Simplify the fraction: 3/6 = 1/2.
- Each fair trial is independent of the last.
📖 Continue Your Journey
Quick summary
- The probability scale runs from 0 (impossible) through ½ (even chance) to 1 (certain).
- For equally likely outcomes: P = favourable ÷ total.
- P(even number on a dice) = 3/6 = 1/2.
- Fair trials don't remember the past — each flip stays ½.
Frequently asked questions
What is probability measured on?
A scale from 0 to 1: 0 means impossible, 1 means certain, and ½ means an even chance. Probabilities are usually written as fractions at this level.
How do you calculate a simple probability?
Count the equally likely outcomes, count the favourable ones, and write favourable over total: picking green from 3 red, 2 blue, 5 green is 5/10 = 1/2.
Is probability on the KS2 curriculum?
It left the England primary curriculum in 2014, but 11+ and independent-school papers still use simple probability — so we teach it as a clearly-marked stretch topic.
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