The average marks in Science for a class of 20 students is 68. Later it is found that the marks of two students were misread as 48 and 65 instead of the correct marks 72 and 61 respectively. What is the correct average marks of the class?

Introduction / Context:
This problem involves correcting an average when two entries were misread. You must adjust the total marks based on the difference between the wrong and correct values, and then recompute the class average.

Given Data / Assumptions:
- Number of students = 20. - Recorded average marks = 68. - Two marks were recorded as 48 and 65. - The correct marks should be 72 and 61 respectively. - Only these two entries are in error.

Concept / Approach:
From the recorded average we find the inaccurate total marks. We then adjust this total by removing the wrong values and adding the correct ones. The net change in the total divided by the number of students will show how much the average shifts. Finally, we compute the corrected average by dividing the corrected total by 20.

Step-by-Step Solution:
Step 1: Recorded total marks = 20 * 68 = 1360. Step 2: Sum of wrong entries = 48 + 65 = 113. Step 3: Sum of correct entries = 72 + 61 = 133. Step 4: Correct total marks = recorded total - wrong sum + correct sum. Step 5: Correct total = 1360 - 113 + 133. Step 6: 1360 - 113 = 1247; 1247 + 133 = 1380. Step 7: Correct average = 1380 / 20 = 69.

Verification / Alternative check:
Note that the net change in the total is 133 - 113 = 20 marks. Spreading an extra 20 marks over 20 students increases the average by 1 mark. Thus the corrected average is 68 + 1 = 69, matching the detailed computation above. This provides a quick mental verification.

Why Other Options Are Wrong:
- 68.5 and 69.5 correspond to spreading an incorrect adjustment over the class. - 70 assumes a total increase of 40 marks rather than 20. - 67.5 would imply a fall in average, which contradicts the fact that the corrected marks are higher overall.

Common Pitfalls:
Learners often forget to subtract the misread values before adding the correct ones, or they adjust the average directly without considering the number of students. Always work through the total marks step by step to avoid these errors.

Final Answer:
The correct class average in Science is 69 marks.