The average of marks obtained in an examination by 8 students was 5.1 and by 9 other students was 6.8. The average marks of the 17 students were:

Total marks obtained by the first 8 students = $8\times 5.1=40.8$
Total marks obtained by the other 9 students = $9\times 6.8=61.2$
Total marks obtained by all 17 students = $40.8+61.2=102$
Average marks of all 17 students = $\frac{102}{17}=6$