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[Original title] There are 50 students in a class. After a math exam, they ranked according to their scores. Results The average score of the top 30 was more than that of the bottom 20 12. A classmate is not clear about the concept of "average" He added the average score of the top 30 to the average score of the bottom 20, mistaking it for the average score of the whole class.

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Solution: the average grade of the whole class has dropped.

According to this classmate's calculation, it is equivalent to dividing the 12 points that the top 30 students get more than the last 20 students on average by. Therefore, it is equivalent to subtracting 6 points from the top 30 students and increasing 6 points from the last 20 students, so the total score of the whole class will be less.

30×6-20×6=60 (minutes).

The average grade of the whole class will also decrease.

60÷(30+20)= 1.2 (minutes).

The second solution:

Solution: suppose the average score of the last 20 is X.

The average score of the top 30 is x+ 12.

The total score of the top 30 students can be obtained from the meaning of the question: (x+ 12)*30.

Total score of the last 20 students: x*20

Then the total score of the whole class: (x+ 12)*30+x*20 = 50x+360.

So the average grade of the whole class: (50x+360)/50 = x+7.2.

Solve x!

Answer: If you calculate according to the method of the wrong classmate:

Average score: (x+ 12+x)/2 = x+6.

Therefore, his average score is lower than the correct average score.

1.2 score is missing.

I hope I can help you. If you have any questions, please thank you in your questions!