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GATE CSE 2025 Session 1 Q3: Class size from a corrected average

Question

The average marks obtained by a class in an examination were calculated as 30.8. However, while checking the marks entered, the teacher found that the marks of one student were entered incorrectly as 24 instead of 42. After correcting the marks, the average becomes 31.4. How many students does the class have?

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Answer: C (official GATE 2025 final answer key)

Concept used: A correction of one entry changes the total, and hence the average, by a known amount.

First step: The total rises by 42−24=1842 - 24 = 18 and the average rises by 31.4−30.8=0.631.4 - 30.8 = 0.6, so n×0.6=18n \times 0.6 = 18.

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Source: official GATE 2025 Session 1 CS question paper and final answer key, organised by IIT Roorkee. Question paper · Answer key

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