📚 Cambridge KS3 Maths: Probability Basics | 概率基础
Probability is one of the most practical topics in KS3 mathematics. It helps us describe how likely an event is to happen, from the chance of rain tomorrow to the result of a dice roll. In Cambridge Lower Secondary Mathematics, students learn to calculate probabilities, use fractions and decimals, and compare theoretical predictions with experiments. This article explains the core ideas and common question types found around page 251 of the Cambridge KS3 course.
概率是 KS3 数学中最实用的主题之一。它帮助我们描述事件发生的可能性大小，从明天下雨的概率到掷骰子的结果。在剑桥初中数学课程中，学生需要学习计算概率、使用分数和小数，并将理论预测与实验结果进行比较。本文解释核心概念以及剑桥 KS3 课程第 251 页附近常见题型。
1. What is probability? | 什么是概率？
Probability is a measure of how likely an event is to occur. It is always a number between 0 and 1, where 0 means the event is impossible and 1 means the event is certain. Every other event has a probability somewhere between these two values.
概率是对事件发生可能性大小的度量。它始终是 0 到 1 之间的一个数，其中 0 表示事件不可能发生，1 表示事件一定发生。所有其他事件的概率都在这两个值之间。
P(event) = number of favourable outcomes ÷ total number of possible outcomes
This formula is the foundation for most KS3 probability questions. The denominator is the total number of equally likely outcomes, and the numerator is the number of outcomes that match the event.
这个公式是大多数 KS3 概率题的基础。分母是所有等可能结果的总数，分子是与事件匹配的结果数。
2. The probability scale | 概率刻度
We can show probability on a scale from 0 to 1. Events labelled 0 are impossible, such as rolling a 7 on a normal six-sided dice. Events labelled 1 are certain, such as rolling a number less than 7 on the same dice.
我们可以把概率表示在 0 到 1 的刻度上。标记为 0 的事件是不可能发生的，例如在普通六面骰子上掷出 7。标记为 1 的事件是必然发生的，例如在同一骰子上掷出小于 7 的数。
Between 0 and 0.5, events are unlikely; exactly 0.5 is an even chance; and between 0.5 and 1, events are likely. Being able to describe probabilities in words is a common KS3 skill.
在 0 到 0.5 之间的事件是不太可能发生的；恰好 0.5 表示机会均等；在 0.5 到 1 之间的事件是可能发生的。能够用语言描述概率是 KS3 的一项常见技能。
3. Calculating simple probabilities | 计算简单概率
To calculate the probability of a single event, count how many outcomes are favourable and divide by the total number of equally likely outcomes. For example, when
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