Key idea: Frequency distribution and class intervals
When data has many distinct values, group them into class intervals (also called bins). Choose interval widths that give 5–10 classes for clear comparison.
For marks 30–98, intervals of width 10 give: 30–40, 40–50, …, 90–100. Each value goes into the interval whose lower bound it equals or exceeds (and whose upper bound it's strictly below, by convention). 40 goes into 40–50, not 30–40.
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