A cumulative count includes earlier classes - Paper 2

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Class intervals: 0 <= t < 10, 10 <= t < 20, 20 <= t < 30, 30 <= t < 40 minutes. Frequencies in that order: 7,3,8,2. Cumulative count at an upper boundary includes all earlier classes. Plot against upper boundaries and include (0,0); individual times within a class are unknown.