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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Solution

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Official answer: C) 30

Quantitative Aptitude

Let $n$ be the total number of students in the class.

Initial sum of marks = $30.8 \times n$
Increase in total marks after correction = $42 - 24 = 18$
Corrected sum of marks = $30.8n + 18$

The new average is given as $31.4$ :

\frac{30.8n + 18}{n} = 31.4

30.8n + 18 = 31.4n

31.4n - 30.8n = 18

0.6n = 18

n = \frac{18}{0.6} = 30

Thus, the class has 30 students, which corresponds to option C.

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This Quantitative Aptitude previous year question is Q3 of the GATE CSE 2025 Set 1 paper. Find more GATE CSE PYQs on Quantitative Aptitude in General Aptitude, or browse all GATE CSE PYQs.

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Quantitative Aptitude has 53 GATE CSE PYQs across 13 papers from 2011 to 2026. 26 of them are 1-mark MCQs like this one, and GATE CSE 2025 Set 1 has 4.

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Independent practice explanation verified by the GateAI team. GATE is conducted by the IITs; question text follows the official paper.