A school has 4 sections of chemistry in Class X having 40, 35, 45 and 42 students. The mean marks obtained in Chemistry test are 50, 60, 55 and 45 respectively for the 4 sections. Determine the overall average of marks per student.

Average marks
$$\eqalign{ & = \left( {\frac{{50 \times 40 + 60 \times 35 + 55 \times 45 + 45 \times 42}}{{40 + 35 + 45 + 42}}} \right) \cr & = \left( {\frac{{2000 + 2100 + 2475 + 1890}}{{162}}} \right) \cr & = \left( {\frac{{8465}}{{162}}} \right) \cr & = 52.25 \cr} $$