The class has 30 students who scored 61 or more marks and 39 students who scored below 61 marks. The average marks of the class is 48.
Total students =30+39=69
Total marks of entire class =48×69=3,312
Let the average marks of students who scored 61 or more be A1 and the average marks of students who scored below 61 be A2.
Total marks = Marks of high scorers + Marks of low scorers
3,312=30×A1+39×A2
To maximize A2, the total marks of the high-scoring group must be minimized.
Since students in the high-scoring group scored 61 or more, the minimum possible value of A1=61.
3,312=30×61+39×A2
3,312=1,830+39×A2
1,482=39×A2
A2=391,482
A2=38
Therefore, the largest possible average marks of students who scored below 61 is 38 marks.