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A pupil's marks were wrongly entered as 83 instead of 63. Due to the average marks for the class got increased by half. The number of pupils in the class is___________?

Correct answer: C. 40

  • A. 10
  • B. 20
  • C. 40
  • D. 73

Explanation

The incorrect entry increased the student's marks by 83 - 63 = 20 marks. If the class average increased by 0.5 mark, then 20 / number of pupils = 0.5, so the number of pupils is 20 / 0.5 = 40. A student may choose 20 by treating the increase in one student's marks as the class size, rather than dividing by the average increase.

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An average represents the total of a set of values divided by the number of values, and a weighted average gives different importance to different groups. Problems involve missing values, combined averages, replacement of observations, and changes in the average, so the total and count must be tracked separately.

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