Question 861362
Given : Number of Boys= 12, Number of girls = 18
        Average Marks of class = 90, Average marks of boys = 87

As we know that Average = (Sum of observations)/(Number  of observations)

Now, (Total Marks)/(Total Students) = Avg marks of class
      (Total Marks)/(12 + 18) = 90
   Total Marks = 90*30 = 2700

Now, (Total Marks of boys)/(Number of Boys)= 87
      (Total Marks of boys)/12= 87
        Total Marks of Boys= 87*12
therefore, Marks of Boys = 1044
Now we know , Total Marks of class = Total Marks of Boys + Total Marks of Girls
Total marks of girls = Total Marks of class - Total Marks of Boys
Total marks of girls = 2700-1044 = 1656

Now Average of girls marks = (Total marks of girls)/(Number of girls)
                           = 1656/ 18
                           = 92
The answer is 92.