If the average marks of three batches of 55, 60 and 45 students respectively is 50, 55 and 60, then the average marks of all the students is:

Solution:
Required average  = ( 55 x 50 + 60 x 55 + 45 x 60 / 55 + 60 + 45 )
                            = 2750 + 3300 + 2700 / 160
                            = 8750 / 160
                           = 54.68