The average marks of 13 students in a subject is 66. If the average of the first seven students is 67 and that of the last seven students is 69, then the marks of the seventh student is:

Correct option is D

Total marks of 13 students = 66 × 13 = 858
Total marks of first 7 students = 7 × 67 = 469
Total marks of last 7 students = 7 × 69 = 483
Marks of 7^th student = 469 + 483 – 858 = 94