Observations misread or wrongly copied: find the corrected mean or the change

This type of question, asked 4 times: CDS 2 2016, CDS 2 2022, CDS 1 2023, CDS 1 2025. All 4 questions below, oldest first.

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Question 1
CDS 2 2016
90 $The mean of 20 observations is 17. On checking it was found that the two observations were wrongly copied as 3 and 6. If wrong observations are replaced by correct values 8 and 9, then what is the correct mean ?$
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Question 2
CDS 2 2022
67 $Mean marks of 50 students were found to be 78.4. But later it was detected that 95 was misread as 59 and 25 was misread as 52. What is the difference between correct mean and incorrect mean ?$
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Question 3
CDS 1 2023
20 $Average marks in Mathematics of Section A comprising 30 students is 65 and that of Section B comprising 35 students is 70. What are the average marks (approximately) of both the sections if it was detected later that an entry of 47 marks was wrongly made as 74?$
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Question 4
CDS 1 2025
100 $The arithmetic mean of 100 observations was 50. Later on, it was found that two observations were misread as 82 and 6 instead of 182 and 56. What is the value of the corrected mean ?$
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Question 1