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15
23−8.
Class 9 · Bangla Version · Mathematics
Data, probability and written reasoning
Use cumulative counts and conditional chance with a clear reference group.
About this lesson and its coverage
Original Kidemy Class 9 progression. This level has its own text, examples and questions. Curriculum versions share the progression; complete board-specific coverage still needs specification matching and additional material.
Learn the idea
Cumulative frequency counts observations up to a threshold. It does not give the number in one interval until neighbouring cumulative counts are subtracted. Conditional probability restricts attention to a specified subgroup.
Worked example
If cumulative counts up to 10 and 20 are 7 and 19, then12 observations lie in (10,20]. If4 of 10 club members cycle, the probability for a randomly chosen member of that club is 4/10.
- Identify the quantities and restrictions.
- Show a method with units or symbols.
- Check and explain the result.
Your written practice
Show your working on paper. Check your answer, then choose Next.
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1/2
6 of the12 girls cycle.
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3/10
6 of all20 pupils.
Write in your own words. A guardian or teacher should review this response.
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One selects only girls; the other selects from the entire class.
Name both reference groups.
Write in your own words. A guardian or teacher should review this response.
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Individual values within each interval are not shown; midpoint calculations estimate them.
Explain the information loss.
Progress and browser storage
Notebook challenge, sources and further study
Apply the idea
Create a two-way table and write two probabilities with different conditioning groups.
Review with a guardian or teacher
- Use the stated evidence or givens.
- Show reasoning rather than only a result.
- Review and explain one correction.
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