Question

The average marks received by a group of 16 students was 20. One student left the group as a result of which the average of the remaining students rose to 21. But another student joined, as a result of which the average marks of the group dropped a bit and became 20.5. What was the average marks obtained by the student who left and the one who joined?​

Answer 1

Total marks recieved by all 60 students is = 60×20 = 320

Now , 1 student left ,

Marks obtained by 15 students = 15 ×21 = 315

Hence , marks obtained by student , who was left = 320 -315 = 5 marks.

Now ,

1 student joined

Marks obtained by 16 students = 16×20.5 = 328

Hence , marks obtained by new student = 328 - 315 = 13 marks

Therefore ,

Required average is = 5+13/2 = 18/9 = 9

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