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Probability in spinner or bag contexts

This is probability applied to a spinner or a bag where each sector or item may represent an outcome.

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Student-friendly explanation

In spinner questions, equal sectors mean equal chance. In bag questions, the count of each colour or type decides the probability. These problems are common because they connect direct counting with a visual model that students can understand quickly.

How to write this in exams

  1. 1

    Start with the exact idea

    This is probability applied to a spinner or a bag where each sector or item may represent an outcome.

  2. 2

    Then show how to use it

    1. Check the model: spinner or bag. 2. Count favourable sectors or items. 3. Count total sectors or items. 4. Divide and simplify.

  3. 3

    Add one concrete example

    If a spinner has 5 equal parts and 2 are blue, the probability of blue is 2/5.

  4. 4

    Avoid this incomplete answer

    A common wrong answer is counting only the favourable sectors and forgetting the total number of equal sectors.

Definition

This is probability applied to a spinner or a bag where each sector or item may represent an outcome.

Example

If a spinner has 5 equal parts and 2 are blue, the probability of blue is 2/5.

Rule to remember

Probability = favourable parts or items / total equal parts or total items.

Memory hook

Count the marked parts, then divide by all equal parts.

Examples and method

Worked example

A spinner has 8 equal sectors, 3 of which are red. Probability of red = 3/8. Equal sector size is the key reason this works.

Method to apply

1. Check the model: spinner or bag. 2. Count favourable sectors or items. 3. Count total sectors or items. 4. Divide and simplify.

Diagram support

Use a coloured sector sketch for a spinner or a count table for a bag.

How CBSE asks it

Questions usually show a spinner, a coloured disc, or a bag of objects and ask for the probability of a colour or symbol.

Avoid common mistakes

Common confusion

Students sometimes count colours or sectors without checking whether they are equal in size.

Common wrong answer

A common wrong answer is counting only the favourable sectors and forgetting the total number of equal sectors.

Exam tip

For spinners, check equal parts. For bags, count the exact number of items of each type.

Quick check

A bag has 4 yellow marbles and 6 green marbles. What is the probability of drawing a yellow marble?

There are 10 marbles in total and 4 are yellow, so the probability is 4/10, which simplifies to 2/5.

Answer writing and exam use

1-mark answer

This is probability applied to a spinner or a bag where each sector or item may represent an outcome.

2-mark answer

This is probability applied to a spinner or a bag where each sector or item may represent an outcome. Probability = favourable parts or items / total equal parts or total items. If a spinner has 5 equal parts and 2 are blue, the probability of blue is 2/5.

3-mark answer

In spinner questions, equal sectors mean equal chance. In bag questions, the count of each colour or type decides the probability. These problems are common because they connect direct counting with a visual model that students can understand quickly. Probability = favourable parts or items / total equal parts or total items. A spinner has 8 equal sectors, 3 of which are red. Probability of red = 3/8. Equal sector size is the key reason this works. Questions usually show a spinner, a coloured disc, or a bag of objects and ask for the probability of a colour or symbol. A common wrong answer is counting only the favourable sectors and forgetting the total number of equal sectors.
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