The marks (out of 100) obtained by a group of students in a Mathematics test are 81, 72, 90, 90, 85, 86, 70, 93 and 71. Find the mean marks obtained by the group of students.

Mean marks (M) = $\dfrac{\text{Total marks}}{\text{No. of students}}$

Total marks = 81 + 72 + 90 + 90 + 85 + 86 + 70 + 93 + 71 = 738.

$M = \dfrac{738}{9} = 82.$

Hence, mean marks = 82.