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The average marks of a class of 30 students is 40 and that of another class of 50 students is 60. Find the average marks of all the students?

Correct answer: D. 52.5

  • A. 50
  • B. 47.5
  • C. 59
  • D. 52.5

Explanation

The first class has total marks 30 x 40 = 1200 marks, and the second has 50 x 60 = 3000 marks. The combined total is 4200 marks for 80 students, so the overall average is 4200 / 80 = 52.5 marks. The likely trap is 50 marks, the simple average of 40 and 60, which ignores that the classes have different sizes.

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About Averages

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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